I hatched some pekin ducks with a chicken hen and I put them in the coop with the adult ducks and chickens around 2 weeks of age. I did put them in an XL dog crate for about a week inside the coop (they free range in the day) just because I was worried about the adult ducks stepping on them. This also gave them time to get to know the others while being safe in the cage. Once they were big enough they didn’t look like they would get crushed under the weight of my adult Pekins, I watched how the flock reacted to them closely. The chickens didn’t bother them much and I find chickens and ducks mostly ignore one another. Watch if you have a drake as he might get territorial but I had very few problems putting ducklings in my coop with the others, even with the drake(he established he was boss and they would keep clear of him mostly). Every flock is different though so just watch how they interact. Look for aggressive behaviour. A peck(one, not chasing the duckling and attacking) if they invade another’s space I find is normal and they learn to keep their distance. Make sure it’s warm enough your ducklings don’t need a heat lamp! The size of your coop plays a big role as well in how newcomers are treated. Every flock is different so just observe how they interact with one another and go from there!