Hello!
Well, for now there are a bunch of reasons why we not adding the Ancients as playable faction, not even in Skirmish. First of all we are still working hard to finish our three main factions so you can play with Tau'ri, Goa'uld and Wraith in other GC's and ground Skirmish maps. This process needs a lot of time. We created the Pegasus Prelude GC to give you something to play in the meantime, because the GC was "relatively easy" to create (we had already the planets, the Ancient models and it contains no ground battles). But to make the Ancients playable as a fourth faction is much more complicated than you might think. Apart from the fact every faction needs at least 10 to 15 ground buildings and a few ground units, new factions won't work in a story GC, because of the lack of extra scripts and commands for the events. Believe me we tried this already. That's why we replaced the Tau'ri with the Ancients just for this GC.
Implementing the Ancients for space skirmish only would work, but nevertheless we then need a few new things (satellites and asteroid mining facility for example). However, the Ancients don't fit to the timeline, after all they are already ascended at the time our mod plays. But don't worry, we got good progress in the ground area and there is some cool stuff for you in the pipeline. And maybe we can think about all this when we have more free space. So be patient