I've herd powder blues can be the terror of the tank but mine is quite docile. One thing that is particularly enoying with powder blues is that they get ick pretty easily. I feed with a garlic infused food and that helps a ton.
As for your yellow tang I would expect there to be some serious turf wars. There was in my tank. I have a 225 with a Sailfin tang, Blue Tang, Yellow Tang, and my Powder Blue. There was some serious battling going on for the first few weeks when I introduced the Powder Blue (he was the last to go in) but they figured it out and fins always heal. If I were to do it again I'd of used an acclimation box for a bit and feed twice as much to get everyone super happy before I released the powder blue.
The stress of reintroduction causes the powder blue to get ick pretty bad so have some garlic on hand and if you run a uv or Ozone on your tank, even better.
Micah
Edited by Sculpin - December 22 2011 at 11:45am