I have 4 betta girls with 8 platies and a bunch of other fish in my 50 gallon tank, and they're fine.
I agree with earlier post, though, I think it depends a lot on the fish, and also if they're too used to having the place all to themselves. I think tank size matters a lot, too, I've had betta girls having no problem AT ALL with guppies in my 50 gal, then when I moved them (same bettas) to a 10 gal once with a few guppies, the guppies were brutalized overnight. I guess the smaller the space, the bigger chance of aggression.