Introducing ducklings is done in stages here.
I hatch about every 2-3 weeks from Mid April - June resulting in about 3-5 groups of ducklings. The first group gets kicked out to the "teen coop" as soon as batch 2 hatches. They have a small house that I wheel out next to the adults. They can't interact, but they can see each other and they get used to their presence. For the first few weeks the teen coop has a heat lamp. As they get older, their numbers grow and the weather improves I remove the lamp.
After the first week the adults then get to come out and and they have supervised interactions in the small fenced in area that the ducklings have during the day. This has usually gone well, and really just ends up with the adults informing the ducklings of the pecking order.
As more hatches happen, the 2-3 week old ducklings get kicked out of the brooder and moved to the "teen coop." The ducklings are way easier to integrate with new ducklings than adults. I usually just do this in the little fenced in area in the AM and let them spend the day together. Usually by nightfall they've integrated.
By the second hatch kick out I've started letting the adults out in the duckling yard during the day with no issues. Once this happens adding more ducklings is easy. There's an initial I'M HIGHER ON THE TOTEM POLE, then they settle in to duck life.
At 7-8 weeks the ducklings graduate from the teen coop and start getting put up with the adults overnight. This is the most confusing part for them I think.. I don't know how many times in the first week I have to take someone out of the teen coop and move them to the adult coop. The ones in the photos above that are HUGE? Yeah those are the 7-8 week olds. It's amazing to think that they're all so close in age in the big picture, but so far from each other in size right now.