it all depends on the jd, some are mean as hell. mine on the other hand has lived with rainbows, geophagus (there was a bit of fin nipping), and now at 8" is in with a 3" jag and pretty much keeps to herself.
i agree that a jag or flowerhorn with additional fish would get way to big for a 55 gallon. you could do a convict tank, or if you wanted you could plant it and do a dwarf tank with many fish lik e abunch or different kinds of rams, max size of 3 inches or so and 10 or so of those would look realy cool in a 55 gallon.
I put my JD in my 55g, he is there with 2 severums, firemouth, and an oscar. It's my temp tank they will all eventually move into a bigger tank. But for now maybe you can get a firemouth for your JD? Or severums?