, I'd take those fish back to the LFS
and start a new stocking plan. Sorry, but if the "brown tang" you're referring to is a powder brown, it's prob going to turn into a bully, the naso and sail fin are going to get WAY to big for a 90, and damsels are just bullies themselves, so if you do restock with appropriate sized fish, they'll bully the new ones. If you keep those fish is a 90, they're gonna get stressed, bully each other to death or get LLE or similar diseases.
And I doubt they'd be very happy in 55 for long together, especially under the stress of treatments! IMO