Thing to remember too is that in the world of cichlids it is difficult to say what a fish will do because there is a lot of variation in behavior. For example, I have a breeding pair of angel fish that do fine (for over a year now) with neons. They are pretty mellow. I had a breeding pair of firemouths that destroyed their tank mates... that is pretty normal for breeding firemouths, IMO. With the firemouths, acara, and kribs as well as others you will get much mellower fish if they are not paired.
What size is the tank and what are the existing fish? Is it the 40 gallon or something else? That is a lot of shark in there.