I think you should go with a community tank with lots of different kinds of fish, not just livebearers. If you want livebearers, I would go with a ram as a centerpiece fish because they are not aggressive (I've had gouramis before- as they get older they can, and sometimes will, kill other fish) and they stay fairly small (about 3 inches, but it depends on the kind you get). That is just my personal opinion. I wouldn't go with a betta because unless you get a female, you can't be positive they won't be aggressive. You won't really be able to know until you put them in the tank. And often the aggressive ones will go after brightly colored fish, such as guppies. Female bettas, however, are also quite attractive.
They take any of my platy fry that manage to survive being eaten by the other fish.