Is your PT someone respectable? I mean, is he intelligent and knowledgable, or does he recommend swiss-ball squats and measuring the length of your poo?
Because if he's an idiot, I wouldn't pay attention to him.
Then again, if he's an idiot, you probably wouldn't be asking this question.
Sumo's put more emphasis on the hammies and glutes, and less on the lower back, IIRC. Also, there's a shorter ranger of motion. The difference is that your sumo deadlift will go up, but your conventional deadlift will not go up as much. Kind of like how, when you do conventional DL's, your conventional DL goes up, and your sumo DL goes up, but not as much.
Once you get a 4-500 lb deadlift, you'll know the differences intimately, and it will matter. For now, probably not. If your deadlift isn't comfortable, try sumo. If it is, tell your PT he's wrong.
OLY lifts are great. Start the powerclean now. There's no reason to wait until you're 'past the beginner stage' unless it seriously just doesn't interest you right now. While your strength will carry over, there's no reason to wait until you have a 1.5x squat before you start doing them. If you plan on getting into oly lifting seriously, the more time you have practicing technique the better.