I can only reccommend mixing mbuna with mbuna im afraid, other fish with similar agression will have different diets, sizes and water parameters.
The only other type of cichlid you might get away with are the haps and peacocks. Also from lake malawi the water that is needed is identical. The haps and peacocks, sometimes called 'open water malawi' can grow much larger than most mbuna and some are piscivorus which mean they eat other live adult fish.
Mbuna generaly speaking need spirilinua/ algae base food where as the open waters really should be fed food more typical of american cichlids, so mixing the two can be difficult.
There are plenty of beautiful mbuna though and really that should be enought to satisfy you. Mbuna are very diverse so there are lots to choose from.
As for bottom dwellers other rift lake fish like smaller synodontis are perfect tankmates. Tank bred Ancistrus are good for algae eating and are hardy enought to take the cichlids and the alkali water.
If of course you ment same levels of aggression within the mbuna and ive completely wasted my time then yeah you are right to some extent. As long as you crowd them agression shouldnt be a massive issue, just dont put in too many males of a really agressive species. Eg socolofi, lombardoi and melanochromis chipokae. I find its best to put the species that have similar diets together as opposed to similar agression.