I'd say go for something a little smaller like a Salvini pair, or Blue Acara pair with some dithers.
I've got a relatively big JD and a Jaguar that's even bigger in a 55 and they get along. I have a couple of stragglers in there as well and they aren't thrashed or anything so I think your best bet would be to go for a pair of JD's
thats alot of fish for a small tank. its not so much if they get along but haveing the CORRECT amount of space for the fish to live. IMO 55 is way to small for a jag let alone a jag and a dempsey.