If the tang is young you may be able to get away with it for a while. If you have pretty intricate rock-work all the better. If it can swim back and forth as mentioned above you should be ok for a while...
..as I said, a while. Once the thing comes of age (1-2 years) it isn't going to be very happy with such limited living space.
You can do whatever you want, that's the joy of being human with self-control -- you can chose to or not to. Most would probably shy away from such... but there's always the chance that his personality is a little refrained (not likely for a tang, but we can always hope) and will do fine in a smaller tank.
Hard to answer a question that sits so high on the fence.
I was also considering this same tang -- but only because I have twice the space 8O I think the fishes' manuerisms are more affected by space than other inhabitants. More inhabitants just places more stress on your filtration -- and more stress on fish really only if some of them are territorial.
My suggestion would be to add the tang last. If you plan on adding one other fish, add it first. The tang will beat up anything new unless it was added before or at the same time.