Depends on the fishkeeper IMO. I had 6 RBPs in a 60 gallon, they were 1" to 6" in size, they shared the tank with 3 tinfoil barbs, bala shark, buffalohead cichlid, cons, guppies and others I cant think of. Oh yeah, a 6 inch ternetzi. Even threw in a gold spilo for a couple hours, but serras own pygos, so I had to take out goldy. Im not sure why the reds didnt attack the other fish, but they left them alone, perhaps it was the amount of beefheart, krill and shrimp I was giving them.
Not recommended for everyone, I had plants and driftwood in particular places. If the Reds were on the left side, I made it so it would be hard to see the right side of the tank. Its like that zen thing in the buddist garden, there are 6 rocks in the garden, but you will only see 5 at all times.