2 nasty trick to try

[Chasm]

Slave soldiers
Step one buy 1 unit of population from gremak pirates to access slave collars
step 2 research slave collars and militia
step 3 after invading a world deploy enough forces to enslave the entire population (both rebels and not) Be certain to have extra empty transports in the assault force, I prefer to have a quantity of my preferred population type to reseed taken/stripped worlds.
step 4 free the slaves, load all the now content population into troopships.
I find this a good way of padding forces and holding worlds.
Note that this works best for races that you do not want to have in your empire ( I use it on the gremak primarily) because others of that races elsewhere in your empire will take issue with it.

Tinker megaworlds
Tinkers do not need food, and ignore morale. They are always content, even if newly captured.
Designate a single large desert / arid / hive world for your captured tinkers (ideally use a whole system if possible). Remove all other population except androids (who also have no morale). As you capture tinkers ship them to the tinker megaworld. Build it with only factories, and keep filling with tinkers. If there are other worlds in the system seed them with tinkers as well, and use them to fill the primary world as their population grows. Use the primary world as your main shipyard. Since there is no population cap you can easily get it to a population able to support all the factories at maximum capacity and produce even the largest of hulls quickly.

[Hans_Lemurson]

I'm a little unclear on what you mean by these.

In the first example, you are using slaves as an invasion-force to create more slaves? Or is this a tactic to quickly remove the foreign population of a planet?

In the second example, I'm not exactly clear on why you are putting all the Tinkers onto the same planet. Why does it make a difference where they are (other than for concentrating shipyard production)?

[Chasm]

In the 1st case it gets rid of all the unhappy population on the planet, while at the same time boosting the amount of troops you have for the next invasion replacing any losses so the answer there is both. No need to build more tanks or whatnot, just use ground troops. In the second case it allows you to get truly ridiculous amounts of production on one world without needing to worry about morale or food. You can literally get to the point of producing anything in 1 turn.

[zolobolo]

Both ideas are nice.

First one regarding slave soldiers is a valid idea - can't you invade with slaves directly though? Do you need to free them beforehand?
In any case: the tactic seem legit as it does have some drawbacks: The invading troops will be of lesser attack value then proper battalions, and the player looses out on production from those pop

I always leave Gremak in place due to their perk to suppress slaves if using the later

The Tinker stacking might be resolved sometime as I think this is in scope in the Bugs and issues thread.
If it isn't fixed, it will probably not be a sure to win trick due to the way how Metal is handled: I always find that this is the bottleneck for my production, and I also always use dedicated production worlds. This end in situations where I need to shot down production from mid-game on some of these worlds just to fit the Metal production capacity. Having a central world capable of pumping out capital ships in 1 turn would not help in my case because of the above + travel distance from the central production world to the front lines.

Now if you would place the Tinker world right next to the front, that would give a serious boost in the war, but s soon as the front line moves, travel time increases again. e.g.: it is not uncommon to have at least 6 turns from my central world to the front line

Lore wise stacking Tinkers might not be out of the question: they do not need food nor shelter so they can literally be stacked on one-another

[Arioch]

Slaves can't fight, no.

[Chasm]

Ok, I guess I wasn't clear. You enslave entire the entire unhappy population. Then free them all (this makes the unhappy population happy until next turn) and then immediately load them all into troopships. This provides ground troops for next assault, and allows you to drop your own population type on the planet to hold it (no need to waste armor units on garrison). You are not loading slaves onto troopships, you are loading newly freed slaves who will fight as militia.

[Arioch]

Ok, I guess I wasn't clear. You enslave entire the entire unhappy population. Then free them all (this makes the unhappy population happy until next turn) and then immediately load them all into troopships. This provides ground troops for next assault, and allows you to drop your own population type on the planet to hold it (no need to waste armor units on garrison). You are not loading slaves onto troopships, you are loading newly freed slaves who will fight as militia.

I understood what you meant. You need an equal number of regular ground forces to enslave them in the first place, and to occupy captured alien planets taken by the combined force, and to keep the militia from revolting if you ever disembark them, but as long as they stay aboard transports, it's true that they won't revolt.

It's an edge-case borderline exploit, since morale doesn't affect population aboard ships and there's a one-turn delay for the morale of freed slaves to kick in, but I don't think it's problematic enough to worry about fixing. You can think of them as penal battalions, if you like.

[akkamaddi]

You can think of them as penal battalions, if you like.

Penal / Slave Battalions is actually a good idea. Bonus to damage, but debuff to defense (you're not going to waste armor on them), and if possible I would suggest they have a "two balls in the hopper" chance (I'm not sure how you handle damage distribution) of being hit over non-slave forces. Basically, Operation Human Shield from the South Park movie. There is a history of this happening in real life.

They would probably require a separate tech and a new tile set, but a color shift/darkening and shackle overlay would probably suffice.

[zolobolo]

You can think of them as penal battalions, if you like.

Penal / Slave Battalions is actually a good idea. Bonus to damage, but debuff to defense (you're not going to waste armor on them), and if possible I would suggest they have a "two balls in the hopper" chance (I'm not sure how you handle damage distribution) of being hit over non-slave forces. Basically, Operation Human Shield from the South Park movie. There is a history of this happening in real life.

They would probably require a separate tech and a new tile set, but a color shift/darkening and shackle overlay would probably suffice.

New card elements for planetary invasions would definitively make that part interesting

Generally we do already have the option to use these penal legions though. Their combat value already reflects their flimsy nature and since I almost never use colonists for attacking (as it is wildly not efficiently), this would be the only way civilian battalions utilized so no need to designate them differently.

It could be argued that a moral indication of embarked pops would be useful but here again the time necessary to implement would outweigh the benefit as normally, pops are migrated from planet to planet directly and thus the value is largely irrelevant

[akkamaddi]

Generally we do already have the option to use these penal legions though. Their combat value already reflects their flimsy nature and since I almost never use colonists for attacking (as it is wildly not efficiently), this would be the only way civilian battalions utilized so no need to designate them differently.

I understand. Actually, I don't think slaves are combat forces.

I was talking about creating an actual military unit, or perhaps converting a population unit into a military unit. If you have slave units on a planet, one of the new units you could research (Gremak get it free) and build would be Slave Battalion. They would be cheap, so you could crank them out quickly. Their benefits from advanced tech would be limited (you only give slaves what you don't need), and they would basically be bullet-catchers for your better units.

[zolobolo]

I was talking about creating an actual military unit, or perhaps converting a population unit into a military unit. If you have slave units on a planet, one of the new units you could research (Gremak get it free) and build would be Slave Battalion. They would be cheap, so you could crank them out quickly. Their benefits from advanced tech would be limited (you only give slaves what you don't need), and they would basically be bullet-catchers for your better units.

I am all for having more types of ground units. Just am afraid this is not possible with the limited resources - tactical space combat being the focus and some of the Harpy mechanics are currently in the works

That being said it would be great to expand upon ground combat as it also has a solid mechanic that could be expanded to incorporate several tiers:
1. Milita/Slave soldiers - These would be drafted from pops like you mentioned. In case of defense, the tech could apply automatically meaning: slaves fight in the defenses when tech is researched otherwise they remain inactive during the battle. In case of offense, militia cannot be drafted for that but slaves could be when the mentioned tech is researched
2. Armored Battalions (aka: tanks). Would work as currently implemented, costing resources but not pop and benefiting from tech research for the combat value
3. Titan Legion - These could be heavy and race specific units either costing a huge amount of resources and/or even bound to specific events. Tech wise these could be end-game tech (to spice up ground combat in the late game) and would not require any upgrades just a specific tech to be researched

Special Battalions such as Marauders and Human Power Armors would be specials with special effects. In case of the existing two these are the Raiding option, but these units could be used to provide combos for the above e.g.: each Marauder could boost a Slave Battalions damage output by +1. Assault

Additionally ad icons, and value for effects for other already existing modifiers: Shield and Assault Shuttles (use the art in invasion screen as well to make it look even more interesting and we have a fleshed out system

Maybe with the next DLC?