I have a yellow Tang and a Yellow Eye Tang in my 55.
I have found from my experience the 6 foot in length rule is just the Tang Police being the Tang Police.
Now having said that, are my Tangs going to stay forever in my 55? That would be a BIG NO! I am working on a 120 (Long, 5 foot) gallon tank right now. A Yellow Tang only requires 100 gallons and a Yellow Eye Tang only requires 75. This is off the LiveAquaria not me!
Saltwater Aquarium Fish for Marine Aquariums: Yellow Tang - Hawaii
Saltwater Aquarium Fish for Marine Aquariums: Kole Yellow Eye Tang
My Tangs are healthy and swim actively, and I know when they get into the Bigger tank they will be even more happy. I got both my Tangs in bad shape as they were both in a 20 gallon tank to begin with. The Yellow Tang was in a 75 gallon, but was getting nipped on and all his fins were nipped off. They took him out and put him in a 20 gallon tank by himself. He was at the fish store for 3 months in that small tank, never growing his fins back and looking worse.
I bought the Yellow Tang and the Yellow Eye Tang because I know my 55 would be better than the 20 they were in. Today the Yellow Tangs fins are all grown back and he has grown bigger and fatter. He is eating, active, and just a lot happier looking. Both look happier than they did at the fish store.
I also knew I had a responsibility to get them into a bigger tank. So I did and I am close to getting it ready for them to go into.
My suggestion to you is NOT to get the Yellow Tang! Not in a 30 gallon tank! That is way to small. But if you ever get a 100 gallon tank, go for it! Many people think they got a 6 foot tank so they are good, but fail to realize all the decorations they put in (Live Rock) take up the swim space, so their fish suffers because they can't swim like they would want. So SWIM space is more important than the 6 foot rule. My 5 foot tank can have more Swim Space than a 6 foot because all my live rock will be spread out and on the bottom. Also remember to give your Fish hiding spots. My tank will have rocks in "V" Shape to form caves for all my fish to be able to hide when they want. My Yellow Tang now loves his little cave he has and it's his sleeping spot. LOL