If you have a really good LFS near by, then you might ask them about a grow out policy.
With one LFS I do business with, I buy babies. My Hippo Tang is the size of a quarter right now. Once the fish grows too big for my tank, I can take it back to the store and they will give me 50% store credit for what they are going to sell the fish for.
This allows me to get a new baby fish or 2 and allows them to sell well established, healthy, larger fish to other clients with bigger tanks.
Thus, I would not have a problem putting a small Yellow Tang in a 55 until it started losing swimming room.