I would say you could also keep 4 in there comfortably but we need to keep them in odd numbers so 3 should do the trick ... get some dithers now in the tank with them while they grow up You can keep lamp eye tetras or black widow tetras in there pecefully a small shoal of them would make your tank do its justice if they will survive... I have seen it done, so it is possible
Yea, I'd put 5 or 6 in. I had three little baby red bellies. The two ate the smallest one and there were two left. After another few months, the bigger one at the smaller one, and I had one red belly. I'd get 5 or 6 because you can always upgrade or sell one.
I dont know what the dimentions of a 65 are so It is hard for me to answer that question. I think a large shoal looks great when they are small but when they start to grow larger, 8"+, I think it looks bad when they are in a small tank, especially a narrow tank. They dont have the room to show what they can do. I think it looks almost as bad as having a large pygo or serr in a 20 gallon. That is just my opinion.