Eh, schooling, shoaling....the point is you need a group. Although 3 or 4 is not ideal, you can definitely get away with it. And someone pointed out pygmy cories. You would be able to stock even more of them as they are tiny. They grow up to 1". If you were going to get 3 albinos/sterbai/jullii etc, you could get 6 pygmies. They are a little harder to find though.