The advice I have read on here is, that you should wait until the duckling has fully feathered.
However, I'm trying my own way. I have two Indian Runners, who are now fully feathered, a KC who is inbetween and a fluffy duckling. The 2 IR's and 1 KC have been together for a couple of weeks now; bit of scuffling in the first day but that was it. The hardest part was hearing the KC pining for me and that went on for a couple of days
To integrate my youngest with the 2 IR and 1 KC, I have had them out together for the past few days free ranging and just getting used to being around one another. By Monday, the youngest will go into the duck house in the night to be with the other three.
The next challenge will be to put those four youngsters into the big pen with my trio. The drake does charge at them when they are all free ranging, but the four youngest have realised if they run under my skirt, the drake backs off The skirmishes haven't ended, but the "attacks" are less dramatic and more of a show than a determined attack.
Two weeks time, they will all be in together and they will just have to sort out their differences themselves; but, that does mean they have had ample opportunity to get a measure of each other first.
That's just the way I am trying it.