I agree if you have corals I would not do anything to the main display. Quarantine is your best bet. How long have you had the quarantine tank? You need to make sure it is stable before you add a bunch of fish. Even then in a 40 gallon to go from no fish to how ever many you plan to put in there could cycle that tank again and put the fish at greater risk. Are all of your fish still eating? I would use Thera A pellets. It is a chemical free anti-parasitic food. I have had Marine ICH (MI) before and I actually floated a mesh bag with Garlic Cloves in it. The water will start to smell like Garlic but it helped me keep appetite up. Cleaner Shrimp will help the fish feel better but does not remove the MI from the system. I have read numerous scientific studies on MI and I believe if you do not add additional hosts to the tank for 8 to 12 months the MI will cycle out. MI can only cycle so many times without new hosts to feed. You can also have MI in you tank and never know it if all of your fish are healthy and stress free.
FWIW I tried quarantine (4) different times over 10 years in my old FOWLR 55g. Every fish I tried to quarantined died. Every time I went back to just acclimating into DT and haven't had a fish die since. People get mad at me and say it's not right but I was just sick of killing fish. I truly feel quarantine is best but they don't work for me. Here is a link to the most comprehensive MI information I have ever seen. It is a 3-4 part article and is very long but lots of good info.
Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro - Reefkeeping.com