you can run into aggression problems in an improperly stocked tank whether its large or small. and it should be noted there is HUGE size variance between different malawi cichlids, they are not all the same, far from it. from dwarf mbuna that only get 3-4 inches to the slightly larger peacocks to the bigger haps, malawi has a huge size variance. 4 feet is not needed for mbuna... it would be for haps like a red empress type thing.
imho just keep 1 male of each species and give him several females, and keep species that are very different looking so they dont fight between species.
its not hard it just takes some researching and the occasional fish may need to be returned if it turns out to be a subdominant male instead of female.
i agree with Don. even in a 4ft tank if you end up with more than 1 dominant natured fish they will between them terrorize all the other fish into the middle. im running into that with my victorians in their 48g. and it doesn't seem to be they r content with their cave, they come out and patrol a good 1.5 ft area. even worse if they r guarding young.
i had that happen when i first started keeping malawi cichlids and i made the mistake of getting them from the mixed cichlid tanks at local stores, resulting in horrible m/f ratios within species. give the males lots of ladies to chase and they will be more content. the males are polygamous harem breeders and arent content with just one female, esp when shes already holding eggs and wants nothing to do with him.
currently i have a dominant pseudotropheus demasoni nesting side by side with a dominant yellow lab, they couldn't care less about each other. they just go off and try to find females of their own species to impress and bring back to the nest. in the meantime the aceii spend most of their time swimming as a group towards the top of the tank.
word of caution though, steer clear of melanochromis auratus, its aggressive as hell even in a 100g tank and since the males are blue and females are yellow they seem to be quite aggressive towards many other species.