Hello Fishman,
I'd say that you need something equally pugnacious and big...like an Oscar perhaps?
I've never kept Jack Dempseys or Oscars, so I honestly don't know how well they would get along.
I currently only have a four-foot species tank full of mbunas, and even though the dominant male is only about four inches long, he doesn't hesitate to attack my hand, or the sponge, when I am cleaning the inside of the glass.
What's that about biting the hand that feeds you?