This not uncommon for sohals. They are very territorial, particularly towards other tangs. What you're seeing is the sohal asserting dominance. This probably won't stop. W/O knowing the size of these fish, I can tell you that you're going to have some problems with crowding and aggression in time, and you're already seeing that. Sohals will grow to over a foot and really need very large systems. If your other fish are suffering, you may consider removing the sohal.