You can keep the tang until it gets larger than 2in. and you can buy some live plants/algae that it will nip at. I have a blue tang and I think he likes that better and I don't ever see him take it off the veggie clip, but someone does cause it's gone every morning.
The problems with having a tang in a 55g are that they will become aggressive when they start to outgrow the tank, they may become stressed and get ick, or die, but as long as your tank isn't too crowded you should be fine for a few years. Yellows need swimming room, but they are tough and I have seen them in smaller with success. Don't let the tang police scare you off. Keep it fed, good water quality, and you can have success.