I have seen a 30gal with a dozen Pseudotropheus saulosi; a dwarf mbuna. It looked great and with no problems at all. The advantage of this species is that you have 2 colors at once: males are blue with dark bars and females are bright yellow. And yes, a 30gal should be a one species mbuna tank, IMO; or maybe for 6 yellow labs and 6 rusties (I. sprengerae). Don't forget a lot of rocks.