I have read some reefers buy 15, eventually also end up with 1 or a pair in the end. Schooling fishes generally school only when they feel threatened. In a tank environment, you might see them school initially, once they settle down, will start fighting amongst themselves. If no big boss (like a sohal tang or something like tt) to bully them, this behaviour may or may not continue. I don't think has to do with tank size or introduced together.