Get another duckling.
First, that duckling will grow SO fast, compared to the chicks, you won't believe it. It will be like King Kong, and can stomp all over those chicks with its webbed feet, within a few days. But it's still best to keep them separate as they grow. Especially since the duckling is SO different from the chicks, and is an only, the chicks naturally will peck at it.
I put 4 chicks and 2 ducklings in the same brooder, but with a hardware cloth (meshed wire) barrier down the middle. They got used to each other that way, but the ducklings quickly grew so large that half of the brooder was too small for them within a week and a half. So, now I have two brooders in my bathroom.
Everybody is a month old now, and the ducklings are about 8 times larger than the chicks. Chicks put all their growing into feathering out at first, whilst ducklings put all their growing into getting bigger. Their feathers don't start right away, like those of chicks.
Ducks need a different type of waterer, and they'll make a mess of it the way they eat their food. This will not work if they're drinking from the same waterer as your chicks. Get a Cool Whip container, cut a slice out of the lid, snap the lid back on. That way the ducks can put their heads into the water but not get INTO it.
I started with a cottage cheese container but it got too small for them before they were 2 weeks old, so I switched to a Cool Whip container.
Ducks need more protein at first, and more niacin in their feed than chicks.
