A 55g is too small IME for a single Jack Dempsey if you get a big one. I had one for over 4 years that was 13" from head to tail when he died. I always get the BS flag on this because I do not have pictures. This was before I had a digital camera and I don't think I even had a cell phone. I was about 16 or 17 when he died so I def didn't. Didn't get one til I was 18. Anyways, a JD if properly cared for will hit 10" and be thick, and produce a ton of waste.
They may be fine together for now, but eventually their territories will be too small for that tank. Minor chasing and nipping is completely normal. If my Midas tank went 30 seconds without a nip or chase I would think something is wrong. I do not remove fish unless fins are being shredded or if a fish is extremely stressed.(hiding, secluding, etc.)