Not sure if it's considered unusual. I do know that when I had 3 Chromis in my system with a Maroon Clownfish pair, they didn't all school together, the Maroons hosted in a GBTA. The Chromis were more or less tolerated by the Maroon female whereas a Blue Devil Damselfish was definitely on the hit list of Mama Maroon. Clownfish & Chromis are part of the Damselfish family, so I guess it's not that much of a stretch that the critters in your system seem to get along so well. Not sure how long you've had all these guys, but at some point the false percs may form a mated pair and I'd say the Chromis will be evicted from the current school.