Well, I was just asking this question recently.
One guy at the pet store said male Bettas should not, but Female Bettas are taking little risk by putting them in a community tank.
I had mine in a 2.5 gallon tank with a Black Moor Goldfish. Now I wouldn't recommend doing that anymore until I find out the problem, because my Black Moor's eye was torn halfway off, but I'm not sure if it was from my Betta or soemthing else.
So all I know is, females are okay with most fish, just don't try any flashy, showy/colourful fish.