I've kept and seen many that keep bettas with guppies in large tanks with a lot of plants. Angel fish, gold fish, other livebearer fish might work as well. Male angel fish can be aggressive afterthey reach maturation but the plants keep them from chasing the bettas. Goldfish can sometime get their fin nipped by aggressive male bettas but they rarely come down from the top to do that and if the goldfish are big enough it doesn't matter that much. Guppies and other livebearer will just out run betta all the time, some babies will be eaten by the bettas but it will not hurt once their population begin to kick in.