FWIW, I have 5 in my 75gal and they are doing great. They do not bother any of the other fish and stay in a nice school about 90% of the time. They are kind of aggressive eaters though. My pair of Maroon clowns and fire fish seem to get their share. Of course I did this before I knew the full benefits of
QT. I was lucky.
In a 15gal
QT I would go with 3 and then 2 or 3 the second round if you really like them. I heard they do best in odd numbers that is why I went with 5. I am always tempted to add more, but then I just remember how big my clowns can get.
If you decide to go with them I think you will be happy they seem to be a great hardy fish.
Good luck,