The inches per gallon rule is very helpful in freshwater but it breaks down quickly in saltwater. Some fish require more space based on activity rather than size. Consider a 3 inch goby and a three inch wrasse. The goby would be fine in a five gallon tank because it will spend most of its life in or near a burrow. The wrasse will be constantly moving and exploring.
Tangs are very energetic fishes that swim great distances every day. They need large tanks to accommodate this lifestyle.
It is also a common fallacy that a young fish is just fine in a smaller tank because you can move it once it reaches a certain size. Fish growth is poorly understood, but we do know that the most rapid growth occurs in the first few years of life - the years when it is small. Sticking a fish in a small tank "because it's small right now" is a recipe for disaster. The fish will naturally tend to grow very rapidly, but cramped quarters will lead to extreme stress and various other factors that will contribute to growth stunting. Stunted fish have shorter lifespans and are more prone to disease.
Then there's always the human equation. If you look at a fish every day, you won't notice the small growth that occurs until one day you suddenly realize its doubled in size, and you meant to give it a bigger home "three inches ago."
A juvenile fish should be housed in a tank appropriate for its adult size. No disrespect intended to anyone, but suggesting a tang of any age is ok in a tank half the minimum recommended size is encouraging a slow and painful death for a beautiful animal.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef