Thanks for the advice! I’ve gone through and reviewed other setups and beginner’s advice for my sea horses, so I’m pretty set up with feeding applications and schedules. I actually use a turkey baster to target feed my scooter blenny in my 55 gal tank now — the darn shrimps are piggies otherwise! I agree 100% about quarantining ANY fish before it goes in any display tank — most of us here probably learned that the hard way at one point in time or another. Your description with QTing a tang is almost exactly what I do with any new arrival. It’s a perfect time to fatten a new fish up, get them de-stressed (I never recommend more than 2 fish in a 10-gallon QT tank, and preferably only one) and generally healthy before addition to a display tank.
I think in this case I was afraid I knew the answer, but was hoping someone would say, "No, go ahead!" Oh well. If I didn’t have the yellow tang in the 55 gal. already I would consider keeping the purple in the pony tank until he got more than 2 inches long, but the yellow and purple tangs have a similar body type and would certainly fight. So for now I think I’ll skip the tang, maybe add a dragon-face pipefish instead. Again, I appreciate the help!