That's exactly it. Overstocked to the point that no one fish is able to be constantly singled out and picked on, they are able to get lost in the crowd so to speak. Now, controlled overstocking is generally used when dealing Malawian cichlids like the demansii or red zebra's. The firemouths you mentioned are Central American and those are bit of a different ball game. In Lake Malawi, the density of fish like Mbuna and Haps are so great, that in a way the fish are used to rubbing shoulders with their neighbor. Territories are at a premium, and they aren't that large when compared to other cichlid's territories. This is the exception to the rule, not the norm.
Not trying to preach you an earfull , just want to you to be armed with the right information.
For mbuna, I'd say around 15-18 would be a good total number to shoot for in a tank that size. The number of males to females is important as well. You want more females than males so that the females do not get harrassed all the time by over amourus males. I'm not sure the ratio, 1-3 maybe?
I love those back to nature backgrounds. But I don't know of a place in the states to get them at.
Backgrounds: Black Jungle Terrarium Supply
This place has some 3-d backgrounds that look pretty darn neat, but are pretty expensive IMHO. They don't flat out say 'safe to use in aquariums' so I might call them just to be sure before I ordered one if I was going too.