I`ve seen it suggested on quite a few occasions that people can keep a male only tank. The first reason being that the males colouring is majoritively better than the females who tend to be a drab brown colour, second being that if there is no competition for the females then aggression is cut to a much lower level.
Unfortunately it`s not always easy to stock a tank with just males, especially if you buy fish as juveniles because a lot of the malawi, mbuna and haps look the same until they reach a few months old.
It helps a huge amount if you buy your fish from a place or a person that knows what they`re breeding/selling, they`re more likely to be experienced with different species and sexes of those species and in turn would be able to advise you better on the sex of whatever fish is being bought