I agree with Mike, but for the opposite reason. Some Bettas can do great in a community tank, but their beautiful fins and slow-moving nature make them a perfect target for more aggressive fish like Angels. And plecos have the potential to reach 18", which would make yours WAY uncomfortable in a 20-gallon. You might want to think about starting a new tank, like, 55 gallons. Then you can also think about getting a few more fish. But make sure it's fully cycled and the water perameters are exactly the same before you even think of moving the fish.