Also important IMO
is that you already have a yellow tang who calls the tank his home. They are known to be agressive. Mean even to newcomers- especially new tangs. Add that to the idea that the powder blue is less hardy, there may be fights for the first days the newcomer is added and you're really gonna be on the look out for any breakouts.
Might have to get a PBT larger than the yellow. Is that possible? How big is the yellow?
Another option may be to remove yellow temporarily and move the rock around.
Or switch. Yellow to QT and blue to main (after QTing) for a few weeks so that the yellow is no longer the first in the tank. Believe me, there could be problems with that yellow and a newcomer tang. When multiple tangs are desired, the yellow is the last to go in.
Another choice.... (hmmmm) take the yellow back whenthe PBT goes into the main (after QT). Get another yellow later.
Or... take the yellow back now, buythe PBT and a new yellow. QT them together and add to the main together.
Choices, choices. Sorry to make it so confusing, but those options are ones that I'd likely consider too. 'Specially since I did have a mean yellow once. Nasty dude to all newcomers.