The Cichlidae family contains over 1,200 species of fish, the majority of which are found in the Tanganyika and Malawi Lakes of central Africa.
They can be quite fascinating and territorial, especially when they pair off and start to spawn. Some will tolerate tank mates and some can only be kept as lone individuals or in very aggressive species only tanks. Some retailers and breeders tend to keep them overcrowded in display tanks to try and limit the aggression and indeed it does seem to work. However, there could be many factors at play in this scenario. The water quality could be very poor, likely high in ammonia (and possibly nitrites) thereby causing them to appear more docile. theory is that a crowding situation prevents a lone fish from becoming dominant over a few. There are just too many other fish to dominate in a crowded tank. Once you get a few of them acclimated and at home in your aquarium the situation could be completely different.
Use caution when selecting tank mates and due your research before getting any of these cichlids to make sure you can meet their requirements.