It depends on a lot of factors, but a 55 gallon tank with gravel and rocks is really not much water, probably only 35 gallons of actual water. And most of the fish from Lake Malawi need much more room than that. Since there are so many different species to pick from, there are 1000's of scenarios. I like to pick fish that don't occupy the same space in the tank. That way there is less aggression over territory.
Fish keeping is also a personal thing, it is hard for me to tell you what to keep. I am better at telling you why it will or won't work. If you want to keep aggressive mbuna types, then you are going to need a much larger tank.