Its always a toss up when adding tangs to a tank that has tangs. The hippo will probably not care, but your Yellow may have issues
Also, the new PBT may have issues with the tangs in the tank. By nature the Acanturis tangs are the nastiest, scrappiest tangs of them all - and the PBT will be boss fish once established
My Achilles took a beating when I first added him to the tank, but he constantly reminds me that its not my aquarium, its his