The powder blue tang IMO is more delicate than others. I would add it first until it is fully acclimated and comfortable. I have 150 gal and I'm hesitant to add it to join my yellow, blue and naso tangs. The powder brown can follow to join in but the remaining 3 (black, yellow and purple) are known to be aggressive with each other. I would add them together at the same time as long as you have ample hiding places so stress can be minimized.