I am currently brooding two different ages of ducklings. Some of my ducks didn't survive the journey, so we got a hatchery reshipment. The first , older group are 12 days old. The second, smaller group are 6 days old. The first group is composed of 6 Pekins, some of whom weigh 15oz already (and one black Runner). The second group is composed of 3 Black Runners who weigh about 2.5oz. How soon can I mix these guys without having to fear the little Runners just getting totally crushed by my big fat Pekins? This is my first year with ducks. Thank you!