Some people do not feed ducklings medicated chick starter, some say amprolium is okay, some feed it. Do some more searching on the forum and go with what you think is best.
Chick starter does not have enough niacin for some ducklings. You can dissolve a niacin capsule (100 to 150 mg) in a gallon of water for the duckling.
How big are the chicks? The first concern is if they are much bigger than the duckling. Ducklings grow SO FAST, that it may become more of a worry that the duckling will be bigger than the chicks.
I would be prepared to put up a miniature fence (I like plastic poultry fence it is soft and flexible and easy to handle) like a wall, dividing the brooder, with duck and its food and water on one side, chicks and theirs on the other.
Ducklings splash water, perhaps more than you can imagine. Please check out tweetysvoice water platform. I intend to make one if I ever brood ducklings again.
Water management is, as some would say, huge. If you can set up a good waterer, it will save you and your birds many headaches, and health issues.
Keep us up to date with what you decide and how it works!
(Think about at least a second duck . . . they like the company of ducks.)