As far as introducing him to the ducks for the first time, I watched him very closely and corrected him for trying to play with them. When our ewes had lambs, it was much the same scenario the first year. Basically once they get the idea the animal is something of yours that belongs on the farm they won't harm it. Our biggest challenge was getting him to accept a new cat. We were able to do it with very short introductions spaced out over a couple of weeks. He loves the cat now, but it took lots of patience.
Bear in mind as others have said they are slow to mature and get more reliable as a guardian as they age. Also be prepared for the possibility of a lot of barking at night. Ours has gotten quieter as he's matured. I think he is quieter because he is better able to determine what is an actual threat and what is not now that he's older.