We keep two mature fossies in a 6' x 2' 2.5' tank with 2 dozen other haps/ mbuna. Very peaceful fish, give them sandy areas to sift thru.
The two we have school all day long. The male does not bother anyone else but the female fossie. I have not owned more than two but think the schooling will be similar, not sure if two males will tolerate one another well.