IMO, according to your "my info", your tank is overstocked as it is. I have had a doliatus and it was quite a joy to have, but is also not suitable for your tank.
Sorry but all of my fish are small and believe me once they cause any kind of problem or become too big I wll have a quick trigger on releasing them, but I do understand that that is just your opinion.
Taking just the question regarding the rabbit, the recommended tank size is a minimum 100 gallon. It is not suitable for a 55 for many of the same reasons tangs wouldnt be.