In my mbuna experience I have found that "overstocking" is the only kind of stocking possible with aggressive cichlids. You have two options. The safest is to obviously keep a fish solo. That can be exceptionally boring though. Add, say a jack dempsey to an oscar tank and one will mercilessly follow the other around and attack it until it dies. Add two parrotfish to this tank, however, and nobody can establish dominance and no single fish would get its tail whooped.