I don't see how anyone can say 100% that tetras will never work with any p. I do agree that with the OPs particular fish and tank size that it wont work but saying it wont work ever because you didn't have success with a different species is just plain wrong. It's like people saying all serras have to be kept as individuals. Macs can be shoaled. I have 6 Macs together. You also "can't" keep serra with pygo. will those 6 Macs are in the same tank as 9 pygos. There are also 2 plecos and 6 tetras in that tank. oh it's a 220g cause I'm sure someone was gonna ask. I don't even have fin nipping problems now that I've gotten past that 4" aggression point. The P's don't even bother chasing the tetras, why would they when they can be fat and lazy while waiting for the their next meal to be handed to them on a platter so to speak.
In general, I agree with not having tank mates for p's. Unless you have the experience with the species and also know the individual personality of your fish, don't just throw random things in to see if they live.
I hate to break it to some of you but the sooner or later argument is based on rubbish. Anything, including any other P, will sooner or later be eaten. Even if you just have a few rbp, eventually the fish will get old and become weak. Unless you have constant view of your fish, you can't really say if your rbp killed one that was weak or ate it after it was dead. So yes sooner or later any tank mate will be eaten but this includes other P's even of the same species.
_-end rant- lol