personally i think with 2 ebjd that you could only add schooling fish or bottom dwellers but not any more cichlids if the tank is only a 55g, that is way to small for 3 fish that get 8-10in IMO, of course most likely the ebjd will not get along once mature (unless they are a pair) and then i think you could get away with 1 ebjd and 1 severum but you would really have to keep up on water changes to keep things pristine