I'm not saying "the only viable strategy".
I'm saying "the only optimal strategy".
As in if you follow it you'll win regardless of map composition, who's around and all that.
We'll need some yardstick as to measure what is successful and what isn't.
Here, I ran a sample game on brutal, 5 opponents, normal map size, normal pace, sparse stars, rare habitability.
100th turn - http://i.imgur.com/BfdlRS6.jpg
- just met the first opponent, Ashdar colonials
121th turn - http://i.imgur.com/dVP2eiJ.jpg
- sure enough, they declare war on me, but I just got gunships developed and buy them on all my four planets
200th turn - http://i.imgur.com/uVHHsG7.jpg
- I'm slowly expanding, while grinding away colonials' fleets; also the humans have also declared war on me, pity; Ashdar imperials to the right were content the whole game
300th turn - http://i.imgur.com/q0U0Upv.jpg
- after I made some advances into colonials territory, I became a bit bogged down, so I settled for peace with them and dismantled humans; as you can see, I already won
360th turn - http://i.imgur.com/73of622.jpg
- I got bored and quit
Up to the late game science destroyers still comprised a sizeable amount of my science, though in the beginning they were about 70%.
I decided not to build any military ships except gunships (because of laziness) and it worked well.
Markets aren't niche, because with an early tech they increase the amount of trade you can do, effectively doubling the income from them.
A lot of income means that I can rush-develop the whole colony, gaining RPs, metal and coins while yours is still developing.
And a large transport fleet means I can afford to evacuate a whole planet if needed.
I don't have a lot of investments into a planet, just money, and money is plenty, so I can be flexible in defense.
I do tend to have tens of science destroyers.
Their cost depends on the pace selected, but on normal it's somewhat in range of 75 PP and 45 metal.
Compared to 300-375-450 for labs it's a bargain and you don't need slots for them. Just churn them out and even if you're locked in your region of space, you don't spend turns in vain.
In the midgame or on a planet with artifacts you can afford to create a research world, but that depends on what the map generator gives you.
Metal doesn't limit rush-production of ships, because there's no penalty.
It's just that you give up the cost of the ship in metal at once instead of per-turn, but the cost is the same.
And the power of rush-buy is hard to underestimate.
While you're building, I'm already ahead.
New planets, whether by colonization or conquest are fully developed and integrated in five-seven turns max.
You have the flexibility (as money and metal go everywhere, but production capacity doesn't) to win the first war in any circumstances (as your science output is mobile and is not reliant on planets whatsoever) on any difficulty.
And then it's mopping up.