Without using anyone's input, I have had them on and off for decades and I see absolutely no reason why they have to be housed in groups. I have had them singly and in groups. They seem to do fine either way.
Actually my experience has been the opposite...........my shoals were WAY more skittish than my single. Once one runs .........they all run. I'm enjoying my single RBP much more than any number of shoals I've had in the past. Granted I might of lucked out with personality.