My friend has a Tilapia around 8 inches in one of his tanks. Its companions include 1 12" bichir, 1 6" bichir, 1 6" JD, 1 5" Pike and a small Sleeper Goby. The Tilapia basically rules the tank and will nip at the JD, but it doesn't bother any of the other fish. Its a pretty interesting fish, though it likes to dig up a lot of his bubblers and mess around with the rest of his decorations. Oddly it has never ever eaten a live fish, even though all the others do. Both of the previous replies were right, he is pretty aggressive and my friend was considering giving his away because it was confining the JD to a minimal space of territory. This has been partially solved by adding caves and territory, so he has settled on keeping it.
Hope these tidbits help!