I let mine see each other, my main flock (7 duck ladies and a man goose), and 2 mallards... not quite sure how old, probably about 5 weeks old at the time.. they ignored each other, or the goose would keep the little ones from eating right away, but it seemed fine. The little ones had the garden, and in the afternoons, I'd put the main flock in the garden too... then they all would sleep together in the duck shed... it went fine for a week, then one morning, I went out and the goose had skinned the feathers and skin off the backs of both ducklings. One worse than the other... so back to separation. still separated, though I added my 4 week old ducks (a cayuga and a runner) to the mallards, since they're the same size, and those 4 are getting along great... not sure how to combine groups though... will wait a while though.