My unprofessional opinion about ICK is as follow:
1. If possible, new fish are quarantined before putting it in your main tank. If the quarantine fish has ick, then go ahead and treat it with any medication or hyposalinity. Unfortunately - I am not always good about quarantining my fish.
2. If for some reason, my main tank gets ick, I leave all the fish in there. I found it difficult to catch some of my fish without breaking down the tank and stressing out the fish. In addition, putting them in a new environment (often smaller) probably stress them out even more.
I have had ick three times. The first time, I quarantined the two new fish in my quarantine tank. They looked fine, so I put them in my main tank. Turned out then had ick, but I couldn't see it in my poorly lit quarantine tank. I ended up tearing up most of my rock work in the main tank to catch the two little buggers. I was unable to catch the hippo blue tang which had been in my tank for awhile, and didn't show any sign of ick. All my fish eventually got ick. The two new fish that I caught and re-quarantined eventually died. I think the stress of catching them ended up killing them. The fish in my main tank were fed a garlic enriched diet, and fed more frequently. All the fish in the main tank survived.
The second time I had ick, it was from another tang I added to the tank. All my fish caught ick from this tang. This time, I decided to leave the fish in and not stress them. Instead, I fed them a garlic enriched diet. I also fed the fish more often. In about 1.5 months, the ick was gone.
Third time I got ick (from a fish that I purchased from some's tank) - I left the fish in the tank. Once again - all my fish caught them. Fed them a garlic enriched diet, and they all recovered.
If (when) I accidently add another fish with ick in my tank, I am just going to leave it alone and try the enriched diet, more frequent feeding, and try to make the environment less stressful for the fish (no major water change). Hopefully the fish will be able to fight off ick on its own.
On the other hand - I have heard stories from very knowledgable people where a single fish with ick ended up killing all the other fish in the tank.
Minh