I have one Pekin that lives with my chickens. I just got 3 new ducklings to go with her. I know chickens will pick on smaller chickens but will ducks do the same? If I separate her when is it safe to introduce them!
I would put them in a cage inside the coop so the flock can get use to them. Then when you do let the babies out to join the flock be ready for them to be picked on, its just the pecking order and the adults will show whos in charge. As for age, play it by ear. Make sure the weather is good, and they are of good size.