IMO, there is a reason a lot of us have QT tanks. And If you have one, why not use it? Thats what you got it for in the first place correct?
I have a Qt for frags and fish. My line of thought is this.... If something did go wrong with a new purchase of either fish or corals
I would also much rather "lose" my QT tank and possibly a few things in it, than lose my DT tank with the thousands of dollars and hundreds of hours I have invested into it.
As for the size of of tank, within reason, I think any fish can be housed a bit cramped for the short time you would need to determine health / Diet preferences.
It has never hurt any of mine. ( I had a Lrg Naso myself in a similar tank and he did great. Also....just my opinion.
Just my .02 worth and I hope it helps.