I had a 30 gal with 4 females in it (And assorted other fish including 6 Panda Corys, a Bristle Nose Pleco, and some Zebra danios) The females all had their own territories and got really picky when the others strayed into their patches.
I wouldn't risk really flashy guppies with most female bettas, but if you wanted to go for something like platys you would probably be okay (Especially if the platy were of equal size or a little bigger than the bettas)