I've read that guppies, platies, and many placid bottom feeders would go well with them. I wouldn't suggest the guppies, though, because of their color.
I've got a male platy, an oto, and a cory with the betta right now, and things are going fine. I also had a female platy and 7 fry before, as well, and they did just fine.
I'd say corys, snails, otos, and almost any livebearing fish would be ok. Just have to experiment...
Maybe the platy you've got in your other tank! He/she's probably already lonely without a companion, so it wouldn't hurt to move it.
HTH