Welcome to AA!
M. auratus are known to be one of the most aggressive of the cichlids, so that's my first suspect right there, but you have quite a bunch of possibilities.
Torn fins are from aggression so I suggest you pull up a chair and sit in front of the tank for a good long while. After they get over their "feeding response" at the sight of you, they'll forget about you and go back to doing what they do. That's when you might see who is chasing whom, and determine the troublemaker(s) in the tank. It is tough to stock a 40gal with Africans, but overstocking might be the only way. Once they are mature it is going to be very tricky (I am assuming they are not all full-size).
Filtration is going to be very important in this tank, as well, with all those cichlids.