I have two duck with 25 chickens and it has been fine for the most part. Ducks are much messier then chickens, especially in the brooder. Drakes can also be VERY randy and can injure the chooks, so you need to watch out for that--one of mine was fine and one had to be re-homed. Ducks also need to be able to immerse their beaks in the water so you'll need to provide at least one water source for them that is deep enough for them to be able to do that. My ducks free range with the chickens and put themselves to bed at night (after dark though, not before) but I've read about others that have to be herded into the coop every night so that is something to consider as well.
All in all I really like the ducks and hope to get a few more this year. They do a good job keeping the slug population under control and can roam the backyard without destroying the flower beds like the chooks do.