i would say that a 150 is too small. They are very prolific swimmers, they need at least 8 feet length to swim.
You culd get a small one, and it will do fine until it gets 5-6 inches. The get way bigger than that though.
Now, the problem with Sohals is that without room to swim they become stressed. This translates into things, disease (usually ich) and very strong territorial behaviour.
I have a clown tang - which is just a Sohal in pajamas, it has over 10 feet to swim, corner to corner, and it does those 10 feet in a second or so. It is also the smallest and meanest of my tangs. She is 3-4 inch. My hippo is 7-8 inch and she beats him up.
--its your call. Personally...I would recommend other tangs, but if you go with the Sohal, go with the smallest you can find and add it as the last fish.
Finally..............the biggest hurdle with tangs is getting them to eat. Quarantine helps them adapt to your food sources without being stressed out by competing fish. ....well worth it in the case of a tang.
HTH,
Paul.