Don't introduce the ducks to the chickens right away. Keep them separated by some chicken wire or similar. And don't do that until the ducks are a decent size. At least four weeks in my opinion.
Practically speaking there are alot of considerations.
Ducks are messy around water. Chickens don't like wet messes. In my coop the ducks stay on the bottom and the chickens have a loft of their own. I do not keep water inside the coop. Which brings me to...
Ducks need water that's deep enough to dip their bills whenever there's food available. Hence no food in the coop. I keep food and water both outside in the run. I have a secure run, however, that they have 24/7 access to. I think that's better for ducks because I find mine outside at night quite often.
I suppose if you had a coop that was big enough you could have food and water in there. Just expect the area to become a mud pit.
If you end up with an excess of drakes you may want to cull them to the point where you have 2+ females per male. Otherwise they may go for the chickens which will end badly. I'm not sure when drakes reach maturity and start getting feisty. I don't think it's before 8 weeks, though if I recall.