corys (minimum group of 3, ideal 4 or more of the same breed), almost any type, are usually ok, they stay on the bottom & out of the betta's way.
Tetras, barbs, minnows & similer small fish are usually ok, though I've had trouble with barbs specifically being nippy, and it does depend on the bettas, don't spend lots of money on something expensive till you have a better idea if they'll tolerate other fish or not.
smaller algea eaters & loaches, though again it depends on the bettas
avoid anything with larger flashy fins such as male guppys or goldfish, and I find fish such as angelfish, cichlids & gourami's to nippy or teritorial or out right aggressive. Either they'll do in the bettas or the bettas will do in them.