I've had my new runner ducklings separated, but sharing a run with the layer hens and campbell ducks for about 2 weeks. There was just a fine plastic net between them and I was tossing snacks right against the net so both groups would actually snack together. I undid the net tonight after the older birds went out to free range and let the ducklings check out the main run. Long story short everyone filtered back in to see what was up and check out the ducklings side of the fence. The big ducks immediately piled into the little duckling pool ignoring their big clean pool. No arguing, no upset. The ducklings were a little put out to have everyone poking in their stuff, but when the older birds got bored and left the ducklings plopped down in the sun and went to sleep. No drama, so much different than the battles that happen when new chickens are introduced.
Well done on a trouble free transition!! I only hope that when I have to introduce my babies to the flock it goes so well. Last intro's made at my place was new adults joining the flock- I kept them in separate pens for a few days before and there was about 20 minutes of scuffling when I removed the fenceline - but all was fine after that.