Provided that the Black Tang is larger than the sohol and has been introduced first it shouldnt be a problem.
If the sohal is larger than the black tang and was introduced first you're going to see a white and red tang.
I introduced rhomboids after the sohal and it punked them to near death.
I had a black tang that was larger than him that I introduced afterwards, it was getting beat up.
Black Tang 5"
Sohal Tang 4"
I got rid of both of them and kept the rhomboid pairs.
Sohal, very very aggressive, he should always be your last addition.
Aside from all that, goodluck dont risk it!
One or the other, Id go for the Black if you want to add more fish, Sohal if you know you're set.