what kind of 20 gallon tank do you have? is it tall or is it long? long would probably better especially if it's mbuna you're looking at. With a 20 gallon, i think one trio of dwarf mbuna would be ok, but that's all you could put. (esp in the case of demasonis, due to their aggressiveness) and when you do pick an mbuna provide lots of caves and shelters. the bala shark would not be a good idea.
or
you could always try getting lake tanganyika cichlids, especially shell dwellers. You could fit them more comfortably in a 20 gallon.