You might want someone with more current experience for Mbuna, but from what I've seen/experienced, Mbuna do not really follow the 1 inch per gallon rule (which is honestly a terrible rule that does not take different species into account).
The advice that I've been given my others and have also seen in real tanks is 2 inches per gallon. Mbuna are very aggressive (again, Labs are not typically) and the more fish you add the better your chances of having a stable, happy environment. If you do not have it already, you need a lot of rock work, frequent water changes and a quality filter with great water flow.