When we started our flock two years ago, I was in the same boat. I read on BYC that it wasn't usually a good idea because the ducks were so messy. But then, I went to the feed store . . . and they were so cute . . . I ended up getting 2 Blue Swedish and one Khaki Campbell duck to go in with my 9 baby chicks.
I don't mean to discourage, but I found that it was not a good idea. From the beginning, the ducks were SO messy that the brooder was a swamp. AND they grew like CRAZY, so the size difference was really a problem. I was afraid that one of the chicks would get hurt or smothered, so I had to set up a separate brooder for the ducks. When they were grown, they were still so much messier and bigger than the chicks that I built them their own house and run.
Unfortunately, I found that I was not the ideal parent to ducks, because they adored being messy and loud and I did not adore those qualities. They were just SO much more mess than my chickens and they were VERY loud, which I didn't expect at all. When they were about 3 months old I gave them to a good friend of mine who has a pond and a special duck pen. They have raised several clutches of ducklings and she drops some eggs by for me every couple of months and I'm much happier with just my chicken flock.
Soooo, in answer to your question, I would not plan on them living together as babies. As adults, if you want to try, I'd suggest making sure that the chickens have a separate, dry place to nest and roost so that they can get up out of the horrendous mess that your ducks will make with their drinking water.
I hope that's helpful! Good luck!