As long as your tank has no live rock or inverts, corals, etc., then you can safely perform hyposalinity in-tank.
With tangs its just lucky or unlucky. If you add 2 or more pretty much within the same timeframe, then they could get along fine. However, as a tang ages, what used to be a amiable tank fish could turn in to a holy territory. Bigger the tank, the better. These fish should not be in anything less than a sparsely populated 75gal or a larger tank. IMO, this fish should be in a tank that can support at least live rock live rock.