As a general rule, you want 1 gallon of water per inch of fish. I've seen cory that are pretty small, but you want to make sure everyone is happy. Two small (1.5-2 inches) cory and a betta should be OK in a 6 gallon tank, the more horizontal area the better., though personaly I would want to give them all more space then that. But if you can only get one, try something else. Cory like being in groups (3 or 4 would be better) and wouldn't be happy at all alone.
You might do better with a glass shrimp.
Last edited by MollyJean; 01-11-2010 at 08:28 PM.