A powder blue tang is also the one fish I've always wanted for my tank, but I'd always been wary of their reputation as ich magnets since I don't have a qt. About two years ago I gave in and got one that seemed healthy enough and that was a couple weeks removed from collection. Sure enough, he got ich within a couple days and took about 80% of my fish with him including a purple tang and yellow tang that had been in the tank over 6 yrs.
I still don't have a qt but I decided that I would only give a pbt another shot if I could find a tank-raised one. It's taken a year and a half but I finally found one just the other day. I'm living dangerously again, as I dropped him directly in my display He'd been thriving ich free in Aaron's (aball011) tank for over a year though so I figure it's kind of like he's been in qt for that long. So far so good, as he's clearly accustomed to tank life and ate nori and pellets immediately. He even knows to come out when I walk by the tank with the jar of pellets. I really think finding a tank raised one like this is the way to go for notoriously sickly fish like these.
As far as conflict goes, my naso pretty much ignored him. My yellow chased him around the first day but has pretty much relaxed since.
Anyway, good luck with finding a nice specimen. They really are about as good looking as fish get.