im sure im gonna get flamed by the tang polizia for this......
but.... IMO, it depends on what your going to be long term.
If your largest tank is a 150, and your never going to go larger, than your shouldnt keep the sohal tang long term.
however if you get a very small sohal tang, and i mean small, like 4" then you will get between 2-3 years out of the fish before it needs a bigger tank or a new home.
the sohal tang gets very very large, so i would say minimum tank would be a 200, but a 150 is pretty close.
live aquaria recommends a min of 180.
again this is my opinion, many may disagree.