Stacey+Tony and I have been pm'ing quite a bit the past couple of days. Judging from the pictures I only see hlle
as being the visual concern along with the regal tang trying to hide in coral and rockwork (irritation burns); however, the behavior of the fishes reported would make one assume otherwise with a velvet sheen (perhaps lost in the pictures), flickering against liverock, erratic swimming, and increased oxygen consumption. My personal recommendation would be to bite the bullet and place all animals in quarantine. A 20g would be suitable for the 3 clowns depending on their size with a 30g being more appropriate. Treatment for them would be Nitrofurazone (Furan 2) or Minocycline (Maracyn 2) for bacterial. The blue tang and wrasse would need at least a 30g, once again depending on size, and the other tang, butterfly, and damsels can be housed in another 30g. The blue tang and birdwrasse require treatment of Cupramine (copper) with a test kit (Salifert) for daily monitoring; however, if ailments get worse then immediate Formalin baths (bucket) would be needed in addition. Basically what this amounts to is treating all possibilities. If the tang and wrasse clear up their velvet sheen and health increases then the same should be applied to the others.
The main concerns are that bird wrasses have poor maneuverability and decent growth, which a 75g does not suffice. Both the regal tang and lipstick tang will eventually outgrow such a system as well and eventually need 125g+. The powder blue tang may suffice in a 75g, but do prefer longer/wider tanks. There are many links to setting up quarantine systems, one of which I sent you, but as I stated before you may not have time to cycle based on the conditions of the fishes you described to me...I'm sure others may chime in on my thoughts and welcomed