Please see game 1885, a captured ancient stargate no longer gets the anywhere in 1 turn feature ashdar imperials get. However it also does not link into your conventional stargate network (scrap the other stargates in the Gienah system to see this)..
Ancient Stargates require the Ancient Control Systems technology to control; being able to build a regular stargate doesn't give you the ability to control an ancient stargate.
I think that capturing an Ancient Stargate is supposed to unlock the appropriate technology, but this may not be properly implemented.