What are you treating them for, are they sick or have a parasite? The treatment to use depends on what they have. If it's Ich you can use Copper or Hypo but if it's Marine Velvet Hypo isn't going to work. If it's Gill Flukes neither Copper or Hypo will work. So it's best to not treat the fish, and once you see that they have something, then start treating with the correct medication.
For Ich I would go with cupremene, The Hypo method is more involved with day to day testing from what I hear, The copper is a little harder on the fish, but as long as you don't overdose or keep the fish in the tank for to long they should be ok.
P...I understand you not wanting to introduce Ich to the main tank but you need to know what, or if, the fish have something before you start to treat. If you do a long QT period, say 4 -8 weeks, you should see at that point if the fish does have something, and if it does, start to treat.
If it's Ich, and you have a setup QT, I would do Hypo. It's pretty easy to do, especially if you have a Refractometer, and doesn't hurt the fish as much as copper would, especially if the fish is very sensitive. Just get the level down to 1.007 and you'll be good to go. Also watch the Ph, as it has a tendency to drop as you lower the SG level.
Like Marrone said.... Determine If the fish needs to be treated first. It may not. There is no reason to put additional stress on the fish. I let all new specimens stay in Quarantine for 6-8 weeks. You have plenty of time to observe and make sure they are free of any unwanted additions...:splitspin I will never not do this again after learning the very hard and expensive way.
I agree, I used to quarantine and treat with copper, but I now just simply quarantine, and let them be. What I have also done that I think is a big plus, is finish the fish off in my 65 gallon fuge for a week prior to introducing into the display. My fuge has a huge ball of cheato, which the tangs really go nuts for. I can also over feed a bit, since the fuge is part of my system and has plenty of filtration. This gives the fish an opportunity to power feed and put on a bit of weight.
Even if they hide for a while when introduced to the display, they can afford to miss a few days of eating.