In my experience it depends on the fish. Ive kept a jag and a JD in a 55 without conflict but then had an oscar and a pacu that hated eachother. Ive kept bluegill with oscars and goldfish without issue. I take it in individual cases just like dogs. I DO know that if i get a nice little school of tiger barbs and then introduce a jack dempsey theyll become lunch guaranteed. Im looking for a larger non aggressive schooling species to pair up with a smaller predatory species.