Female bettas are not AS aggressive in most cases. We've had 2 that had to be by themselves. Our first one got ahold of a neon tetra and shook it like a dog shakes a rag toy. Our newest female beat up the male when we tried spawning them and has killed 2 otos and 1 julii cory. (Bet that hurt our male's pride) Now the 2 we have in our community 10 gallon are doing fine. They're in with platys, ghost shrimp, guppys, otos and corys.
From what I'm seeing size in the female might have something to do with it. The blue female we tried to spawn is a big girl. As shown in my album for the 5.5 gal betta tank. The girls in the 10 are little. The platys are bigger than they are! So perhaps starting them small in a community tank gets them used to other fish. I know both the aggressives ones were from Petsmart and stored in those horrible cups. While to nice ones were from my real LFS and were in a tank together. Already working on that sisterhood.
But this is just what I've observed, mileage may vary.
