Putting Tang in 20g QT, won't it stress fish even more?

[alve]

I will be setting up my new 156g Oceanic Reef tank in two weeks. Everything from my existing 100g tank will be moved over. I haven't added any new fish in over 2 years and I do not want to add anything bad into the new tank so I would like to put every new addition in a 20g QT before introducing.

If I eventually put a yellow or regal tang in a 20g QT for 4-6 weeks before adding to the display tank won't that stress out the animal?
I know they need a big tank and a month in a 20g seems to me that it would stress out the animal a lot?

After a lot of reading here on RC it looks like the ideal is putting fish in a QT for 4-6 weeks. Doesn't ich or any other disease show up sooner like after a week or two? Wouldn't it be ok to add the fish to the display tank if everything looks fine after two weeks? It still sounds a long time for a tang being in a 20g although they might have been in a tank like that for a much longer time at the LFS?

Also, do you medicate fish as soon as you put them in the QT or only when they show signs of ich or any other disease?