I really don't think its a mimic tang; they don't have the white ring at the base of their tail like yours does, and have a dark dorsal and anal fins, where your's has a yellow dorsal fin and dark anal fin. If it were a juvenile mimic changing to an adult then I think the anal and dorsal fins would change at the same rate, and the body color would be lighter than the face color. I really don't know what it is at this point, maybe a juvenile dussemieri or black spot changing to an adult? This might be an instance when we just have to wait for it to get bigger! I really wouldn't worry about it at this point since you are getting a 150g (is it the 6ft long version?) - even if it is a larger tang, it will be fine in the 150 for awhile. And by the time it outgrows it (if it does), you will have a better idea of which tang it is. If all else fails, send the pics to wet web media and see if they can help....