It could be done in the way that starcraft was done.  They had massive battles but didn't have leaders in them.  Instead they made a couple levels where you had to take a small squad around a space station and do certain objectives that way. 

You would also rescue other forces to join you group to add to your fighting strength.

If they did that with PoR, Arbiter, and Brute I think it would be crap and iam going to commit suicide.This is a crap idea and iam a crap member and iam going to rape my *** brother