The reason why we say no to tangs in small tanks is that tangs like to swim around alot and they do not tend to be happy in smaller tanks.
now, to answer your question "Can a Yellow Tang live in a 30 gallon?" Yes, but for how long, i don't know. And the tang may not be happy since he won't have much room to swim.
In the event you do get a tang, remember they need to eat green foods. Also, they tend to be prone to illness so be sure to qt.