Two factors will determine if a foxface will go with a yellow. 1--What size tank are we talking about? 2--Since your tang is so large already, and assuming that there is room for a foxface I would still be leary of doing this combination. Not because they don't get along, they will, but the yellow tang is an extremely territorial fish, just plain bully.
If you still want to try it I would definately qt the foxface, and the tang. Not together. You need to get the tang out of the tank (his turf) and place him somewhere else for at least 10 days. Maybe rearrange where he hangs around too.
Good luck, but yellow tangs are bullies and usually are the last fish added to a tank for this reason. Changing his surroundings for a while will help things out. But may not ultimately work either.
Oh, and get a foxface that is at least 3".
