I used to think the exact same way until I had to deal with a major ick outbreak. If a fish is put in a QT tank that is smaller or much smaller than recommended, surely he will not be as happy but sometimes it's a lesser of two evils. Bring things to a head now in a smaller tank where it is more manageable or later when the entire tank is at risk.
A large fish can live just fine in a small tank for a few weeks in Hypo to let the ich die off. If you keep a 7in fish in a small 26 g with no treatment such as Hypo, sure he will be stressed and the ich might get the better of him but the same fish in a 26g under Hypo treatment will be, arguably, less stressed due to Hypo itself and the free swimming ick won't be able to live and reinfect the large uncomfortable fish. You may get lucky for a while but ick will eventually get ya!
I agree that, in a perfect world, no QT would be needed as long as all the fish are healthy and can fight off the parasite but it's not perfect and will eventually come back to bite you and your fish in the butt.
Man, I used to believe the old wive's tale that ick was always there and if the fish were happy and well fed, they would not get ick. That was true until the first time I did a re-aquascaping of the tank and scared the Regal Tang and then the rest of the fish got ick as well.
If there is no ick present, it doesn't matter how much you scare the fish, they won't get ick because it won't be there to infect the fish.
If ick were a single bullet in a gun, would you take the same chance putting that gun to your head and pulling the trigger as you would putting a healthy looking fish in your main display tank?
This one is a touch and go I have added a tang to my tank w/ out qt. It was a Large (7+") Naso streamer I had a qt of 26G I really thought that Id be better off putting him in my display vrs stressing him out in such a small tank. Id say unless you have a 75G+ qt tank he was better off putting him in his display from the get go. IF its a healthy fish from the get go more than likely they shouldnt be any probs. NOT to say you wont have any. Best of luck.
