How long has the tank been up? Good job on your part, quarantine tank is definitely the way to go.
I love powder blue tangs but fair warning they can be problematic with ich, be prepared to quarantine and treat. I wold lean toward a hippo tang to avoid frustration. Yellow tang, definitely go for it. I also like yellow eye kole tang and bristle tooth tomini both compact and built for cleaning rock.
Do you have any interest in blue green chromis. I love their simplicity, they are affordable and half dozen or so look great on a reef.
Blennies have great character. I like bicolor and sailfin algae blennies. While most fish just go about their business, you'll observe these guys watching you as much as you watch them. Neon gobies they say are good for parasite control.
I'd shy away from a copper band butterfly initially. I think they stand a better chance in a mature tank down the road and you may be better equipped as an aquarist to meet the challenge. As far as acquiring one, look for one from Australia. I can assure you they hold up better than ones sourced from Indo or Philippines. I would not personally waste my time buying one from anywhere other than Australia. I'n not saying it wold be impossible to keep one from elsewhere, just that I believe your odds would go up significantly by obtaining an Australian sourced cbb. More money too!
Hope that helps.
Do you have a tank thread where you post about your acquisitions?
TR