PB is likely going to be a problem. Mine is at least. Can be done. There is an article on tang aggression on here somewhere. Read it. learn it. Use it. GOOD LUCK!
I would not recommend it in a 100 gal. And both fish will need to be rehoused in the long term. 125 gal long minimum for Powder Blue; and 150 gal long minimum for Regal Blue. Ideally 180 gallons 6 foot or longer / larger for both. Tangs are a very active fish.
I would say wait until you have an adequate sized system, as the way it sits right now it is housing Tangs too large for it. Adding for is asking either for aggression, illness or both
Chap is new to R2R …. ask the mods to consolidate the two (identical) threads and move to the fish forum. You'll get more views/opinions there. Agree that the tank is tight already for a hippo/regal and powder blue. Probably not a great idea to add a third. Powder blue has a reputation for being on the aggressive end of the tang spectrum; not clown/sohal aggressive, but comparable to the purple..
Both of these Tangs, I’d like to add, are not on the very disease resistant end of the spectrum for Tangs. Moving them to a larger tank will reduce the chances of disease and stress for themselves and for other, more passive fish.