tetrin
Aquarium Advice FINatic
mumba's +
shelldwellers (shelldwellers will likely be killed by the mumbas by the way)
ps lanisticola is actually a mbuna, it just loves its shells. it is also one of the most peaceful mbunas.
i totally agree with greenmagi that a single species will work in the tank, but its just a waste of space in a 36*18 tank.