I integrated my ducks and geese the same way I did my chickens. I separated the house in half with the babies in one side and the older duck and goose on the other side. The babies had their own food and water, and were locked in where they could see and be seen, but not be touched. I kept them like that for 3 weeks, so when they all got mixed the babes were about 6 weeks old. At that point I pulled out the partition and had not a single problem. The flock integrated beautifully. I was worried about my goose more than my duck. The goose had initially tried to attack the babies when they were first put in the house. Frenchie is huge compared to the babies, and could do a lot of damage had she been able to get to them.
I had to move the babies outside at 3 weeks because I couldn't stand the mess they were making in my house, although they were not fully fledged at that point. I'm not sure where you are at, but I usually try to wait until babies are fledged before putting them outside because of my rather harsh weather.
Hope this helps. Good luck.