Any Tang can possibly become a risk. Awhile back, I had a Lavender Tang, which I really liked, develop an appetite for Acans. Tangs are a "grazing" fish and need to be able to eat, whenever they want, throughout the day. Otherwise, they may take a liking to corals. If you don't have a natural source of algae growing in your tank, they'll need to be fed multiple times a day, with an algae clip.
Tangs are also animals that need a lot of room to swim. Along with the size that these guys get, a large tank is necessary, even at a smaller size, to allow them the room to swim. I often wonder if my tank, which is a 200 Deep Dimension, is large enough, since it's 4' long. But with the 3' front to back, giving them 12 square feet, I'm comfortable with Tangs in there. Otherwise, I'd suggest they always be in a 6' tank. Sounds like you'll be upgrading to a 6' tank soon. Sailfins are amazing Tangs. Be careful, adding one now, if you ever plan to add more Tangs later. Sailfins are among the most aggressive, along with Powder Blues, Clown Tangs, Purple Tangs and Achilles. You may have better luck, if you ever plan to have more Tangs, to add the lesser aggressive types first.
