I would definately add the powder brown first, wait for it to adjust then add the sailfin. Sailfins can have a nasty disposition to new comers. Not all, but they are the stripped version of the yellow tang. My sailfin is definately the king of the tank. Any newcomers and he is the first to let them know this!!
If putting them in together, you should be fine. No time for the sailfin to make his territory.......the entire tank. The powder brown is typically a sissy fish, not the dominary one. Which is good to pair them up.