I have 2 ducks in my chicken coop. The first because she imprinted on chicks. She learned to climb the steps to get to her friends in the coop instead of following her own kind to the pond. I added a Drake recently. I free range and keep food available at all times. This is the first I had heard of ducks overdoing it. I seldom see the ducks by the feed when the pond is open. My duck hen is just one of the flock the Drake is a khaki and tends to just stay out of the way. Remember ducks need head dunking water. This doesn't seem to be a problem for grown chickens, but be careful with peepers. Also if you have a pond getting ducks into shelter of any sort is a challenge. My chicken imprinted duck is the exception, luckily my current Drake wants to be where she is.
No water in the coops, because ducks will make a giant wet mess around their water. I just keep kiddie pools and water buckets outside in the run for everyone.