I had a male betta in a 10gallon along with 3 female bettas, 2 dwarf gouramis and 3 corys, and had no problems. This situation could be case sensetive however, I have had bettas that were completely mellow and were no problems in community tanks, and other bettas with an eternal chip on their shoulder, who wold attack anything and everything in the tank with it. Most likely though, I think you could probably house bettas and corys together without any unfortunate incidents.
Caveman