I got this answer when posting this question on Y!A:
"With Malawi cichlids, it is not the size of the tank or the number of fish, but the size of the tank bottom and the number of males that primarily determines tank size. Mature males claim a territory that they are genetically predisposed to have a certain size. We can only modify that so much with boundary markers, rock work that creates a visual barrier between parts of the tank.
You have one of the more laid back species of Malawi cichlid in the L. fuelleborni. They are better than average at eating algae and benefit from tank conditions that grow algae on the rocks and back of the tank. They keep it more closely cropped than most other Malawis do.
I first bred the M. livingstonii when they were using the name lanisticola. The young and juveniles may claim empty snail shells as a refuge but the adults are too large to fit inside a freshwater snail shell and move on to crevices or caves in the rock work. I never provided them with shells or live plants other than hair algae and Najas guadeloupensis (called "guppy grass" or "cichlid candy").
If your fish are young, removing extra males as they appear is a good idea. The first male to mature is usually not a good choice to breed. By being sexually precocious, it may not reach the full size and full coloration the species is capable of. To protect the quality and robustness of the fish I breed, I always remove the first few males that mature and use later maturing males as breeders.
Watch out if you have an especially dominant male of either species. A dominant male will breed with all the females in a tank regardless of their species, resulting in hybrid cichlids. As the fish grow, eventually they are likely to find the tank seeming smaller and the chance of a dominant male increases. At that point you might need another tank to keep crossing from happening. However if you do not plan on raising any of the fry, then it is only physical competition you need to watch out for. In general six foot or two meter wide tanks are better for keeping rift lake cichlids, but I have bred many species, one per tank, in four foot tanks 75 or 90 gallons. The 55's don't provide much room for creating rock backgrounds and caves."