For an African Rift lake cichlid community tank (Malawi or Tanganyika), the standard is that it contain adequate shelter (e.g., caves, hiding places), unless it houses mostly a single species and/or is heavily stocked. Malawi mbuna aggression is no secret but it is often underestimated and underappreciated. Tanganyikans can be quite similar, but I've found that abundant shelter in Tang communities makes a huge difference. In my experience, mbuna are more apt to "carry the fight" in an aquarium, which means aggression isn't as greatly mitigated by shelter as it is with Tanganyikans. I have never kept cichlids endemic to Lake Victoria, west Africa, Madagascar, etc. so have no experience to draw upon, though there is no reason to believe similar shelter issues don't apply.
My current Tang community tank contains 8 species of cichlids, 16 total adults of different gender ratios, including some singles (there are some fry and juveniles too). It's a reasonably heterogeneous community. For more information on the species and ratios, see my post from back in November, What is my set-up, Pt. 1.
Though my mbuna tanks and Tanganyikan tank are all heavily loaded with rocks, I've found that the size of the caves formed by the rocks can provide a significant advantage. Recipients of aggression of any type (breeding, territorial, status) need a place to retreat to in which the aggressor has no access. Sounds profound, but you would be surprised what little attention is typically paid to the actual size of the crevices and "caves" that are formed when the rocky aquascape is redone or initially done. Cichlidophiles are well advised to pay closer attention to that.
Make no mistake however, caves and other hiding places can be created without rocks or ceramic products designed for cichlid breeding, etc. If tank ornaments are what you like (ships, logs, castles, etc.), they can provide sufficient shelter too.
As part of my Tang community set-up, I use customized ceramic caves made specifically for me by Pleco Caves in Indiana. I posted about Pleco Caves back in August, 2016 where I described a custom order I worked with them on. In fact, I just ordered some new ones with new dimensions that had been molded as of last week but still need to be fired. The advantage of these is that I don't have to be as cognizant about my rock formations when rescaping. The new caves have been designed to perfectly accommodate specific species and exclude others. I am very confident this is one reason why I have not lost a single cichlid to aggression in my Tang community tank. So, in a nutshell, shelter size can matter. It's not really shelter from aggression if the aggressor can still reach the defender.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub