Im almost certain if those dempseys the terror and oscar are all in that 55 gallon by the time they are a good 7 inchs or more, there will be a nasty situation on your hands. You could get lucky and possibly not have this happen, but thats very common with that small of a space and that many large aggressive cichlids. They will be fighting for territory. Right now at 2.5 inchs they aren't gonna be scrapping about in a 55 because its plenty big for them at the moment, but that could quickly change in the months to come. Ask anyone on this board and im sure they will tell you that a 55 gallon is only big enough for one of those cichlids let alone all of them combined, and then as for those others such as that little pink gourami, may god have mercy on them lol my guess is, the fish that will be left remaining are the 2 dempseys if not 1 and the pleco