Ben South BvetMed MRCVS from Chicken Vet says:
Ducks, geese and chickens are perfectly happy to coexist and I would introduce them like you would a new pair of chickens. Quarantine them for two weeks in a penned-off area where they can see each other but not have physical contact. Monitor them closely for another few weeks to ensure that there is no aggression occuring.
Ducks do require different environments, specifically their housing requirements. They need a safe place to be at night, away from potential predators as they don't generally perch to roost. They will sleep on the ground and require shelter that they can walk into without injuring themselves. A wide ramp and pop hole are ideal and this can be shut at night to provide safety.
The housing must have enough space for all the birds to be comfortable. Ducks and geese won't roost under chickens, so the floor space must be adequate if they are to share the coop. You will have to train the ducks initially to use the accommodation and this can be done by keeping them in for a few days and then having a limited range around the house for a further few weeks.
Like chickens, foraging is important for ducks too. They will graze a lot more than chickens and will nibble grass. Obviously access to a body of water is essential to allow the birds to clean and preen. Keeping access to the pond clear and clean is advantagous, and having wooden slats may help.
Another difference in husbandry is feeder types. Ducks require a wide feeder which allows them to fit their bills in without hitting or damaging them. I would also recommend monthly faecal examination for worm eggs, and the same treatment should be given to ducks, geese and chickens simultaneously.