I've known dozens of people who've kept chickens and ducks together with no problems. Farmers have done it for a long, long time. Centuries, in fact. Drakes can be rowdy-- they'll try to mate with anything if there are no ducks around, including other drakes, roosters, dogs, whatever moves...
The ducks will need time to settle down and figure out that the coop is home base, so I wouldn't let them out for a while. The best route is to keep them separate in the run for a while, so they can look but not touch. If that is not an option, then just keep an eye on them when they're put...
So long as there is enough room, it should be okay. I would introduce the ducks slowly into the flock. Keep them in a smaller wire enclosure or cage for a few days in the coop to let them get used to each other, and then try having them all together. If there are no problems, you're set. If...