Mmm, I've kept singles and they've been ok. If you want to keep a school get a big tank, lots of water movement and keep at least 6 or 7. And get used to feeding them 4 or 5 times daily.
The tougher ones tend to be terribly aggressive. For some reason I can get them cheap as damsels, but they routinely come out of shipping in a terrible state, indicating they have a strong need for especially well oxygentated water.