Yes Purples are mean, they should be added last. I would wait until your 210 is up and ready, transfer your Angel and Sailfin and use your 90 to quarantine your new fish. [ Thats what I would do anyway]
Powder Blues are not hardy fish, yes a lot of people have luck w/ them but most do not adapt well to captivity
Not sure which 'Powder Brown' you mean...Ancanthurus Japonicus or Ancanthurus Nigerious [sp?]. Of the two, Japonicus is the better, more hardy fish. The other does not adapt to captivity, often dying in the first month. I have a Japonicus, beautiful fish... ich showed up 2 weeks into qt [of course] but it has had 4 weeks of hypo and is doing well.
If your 210 is a 6 ft tank, I would probably only add one more large fish. Sailfins and Purples are of the same genus so there is a better chance of aggression between those two . Its really a toss up, A Powder Brown maybe harassed by the established Tang where as the Purple added last may end up the aggressor
So, I guess I am really not being much help

I would say either but I would only choose one