Getting an Area 51 proposal into the Commitment Phase is difficult.
Getting a proposal from Commitment into private beta is very difficult.
Getting a private beta into public beta is incredibly difficult.
Getting a public beta to graduate is almost impossible if you are a niche site like Robotics (it looks like this site will be in permanent beta for instance).
Robotics even had the advantage that it was originally part of Electrical Engineering and received a lot of support from that community when it was decided that they didn't want Robotics as part of their scope.
Meanwhile, proposals for the maker/3d printing/hackerspace community got into beta twice with Personal Manufacturing and my Digital Fabrication proposal, but both times had to be closed due to lack of activity.
For the moment, I would give the FIRST Robotics community the same advice that I've given the 3d printing community. Post your questions here with appropriate tags until you can get your own community off the ground. If a FIRST Robotics question about is appropriate for any stack exchange site, then it will most likely be appropriate here. If it isn't a good stack exchange question (because it is too broad, unclear or primarily opinion based) it wouldn't be appropriate in a FIRST Robotics stack exchange either.
Also, consider that the more people you bring to stack exchange in general, to get the feel for how it works, the more people will be attracted to your own proposal, and the more people you will have with 200+ reputation, able to help get FIRST Robotics off into beta.