To ease the confusion a bit (at least from one perspective) all lake malawi cichlids can theoretically cross with each other, whether they belong to the same genus or not or same group (mbuna, hap, peacock) or not.
Howver, pretty much all of the species are 'wired' to seek out their own kind, especially if the opposite gender of their species also exists in the tank.
Even when their own kind isn't present in the tank, they may not attempt to cross with other species. I have a few lone species representatives in several tanks and they have no procreation-related interest in any of the other fish for several years now. With that said, some individual fish are more apt to cross than others.
With proper gender ratio's in place AND the absence of similar looking females, species-crossing or interspecies breeding right competitions will be significantly reduced.
Rift Lake and Reef tanks.