I have a normal Jack Dempsey, and it is one awesome fish.
He is very territorial and intollerant. He usually will not nip any fins but will chase anything that gets in his way. The only fish he will hassle is a 5" goldfish, he seems to just hate that thing.
We did try to put some neons in the tank... we got 12 for very cheap and they lasted about 6 hours before JD had eaten all of them.
He will be going into a seperate tank in about a month. The only fish in the tank that are'nt intimidated by him are the Pacu (who is almost 20" now), the pleco, and the blue crayfish. Watching the JD and crayfish have a mexican stand off in the middle of the tank is always enjoyable!