I brood my chicks and ducklings in the coop using a mama heating pad and/or electric hen (depending on how many there are). They are separated by wire so there is no physical interaction but they are in the coop while still just a couple weeks old. I believe it's much easier to integrate them while very young because they aren't a threat to the older birds. After a couple weeks, an opening is made and the chicks/ducklings can come and go. I haven't had any injuries and no one seems stressed.
If you can't brood in the coop, I suggest introducing the ducklings early and bringing them outside often. Keep them separated by wire so no one gets hurt but let the older birds come and inspect the ducklings. By the time the ducklings are old enough to go outside it won't be as much of a shock to everyone. Keep them in the coop but still separated for a week or two. The length of time will depend on your flock. Then just let them out and watch.