i know what you mean, no matter what lfs or chain store, there are assorted or mix mbuna.. however sometimes i look at them and i see distinct genus.. like the other day i was looking at a mix cichlid tank, but i could tell that one of them was a giraffe hap.
If you want to make a Malawi Tank, i'd avoid getting two species from the same genus....you might just end up with mixed cichlids. what i mean by this is.. don't get two from pseudotropheus genus.. honestly it's all up to you. if you want mixed cichlids and are fine with that.. then go ahead. The only problem is that you'll have a hard tmie trying to find compatibility between your fishes and if they breed, that makes the problem worse...