My theory is that each chicken or duck has it's own personality and each flock has it's own dynamics. You can change those flock dynamics by adding or removing one flock member. So individual personalities and flock dynamics plays a part, not that one flock is hens and one is ducks and pullets.
I almost always have a mixed age flock. Often I have a broody hen with young chicks being raised in a flock with an adult rooster, adult hens, and chicks and adolescents of various ages. No ducks, though I have raised turkey with them a couple of times. I hardly ever see a chicken of any age go out of their way to attack the chicks. When I do it's usually some young cockerels and Mama makes short work of them. But occasionally a chick will leave Mama's protection and invade the private space of another older chicken. With some older hens this is not a big deal, they ignore the chick. But sometimes they peck them to send them away.
Something I've seen happen several times, usually when the chicks are about two weeks old. The chick leaves Mama and goes to stand around the feeder with adult hens. Sometimes the other hens ignore the chick but usually it's not that long before one hen pecks the chick to remind it that it is bad chicken etiquette for it to eat with its betters. The chick them runs back to Mama squawking and flapping it's little bitty wings. Mama usually ignores this, the chick has learned a valuable lesson. Don't pester the older hens. But if a hen chases that chick to further that lesson, Mama goes full broody hen and whips butt, no one threatens her chicks.
You have my sympathy with that fence, I've been in that situation. Some things you learn through experience. Are you using wire mesh or some type of netting? With the netting I've had chicks get in through the end if it is not sealed. It's amazing what they can squeeze through. With wire mesh I made an apron, bending the bottom of the mesh 90 degrees and laying 6" or so flat on the ground so they did not squeeze through under the mesh fence. I don't know what you are planning or what your fence looks like but I sure wish you luck on that fence.