There are definitely going to mixed reviews as we have all had different experiences with this.
I have raised them both together and apart havent had issues either way. If you have multiple ducks they dont try to mate the chickens(at least thats my experience) I just recently had 2 baby ducks with 4 chicks and the only reason i separated them (after about 3 1/2 weeks) was the chicks kept flying out when my kids would take the ducklings out to play.
However, I no longer house them in the same runs/coops when they are older (To many of each) but i do raise them together most times until they get so big that the ducks need more and more water and more room to roam. I have also never had a duck or chicken think they were one of the other breeds.
Yes, they can trample the chicks as they get bigger... if they a pretty big age gap. Even with that said each situation is different, each animal is different and can really depend on the Temperament of each bird. And you do that to keep in mind that ducks grow extremely fast and get very big quickly. So they will more then likely need more room quicker then the chicks will.
Depending on were you live, building an outside brooding run for them to spend their days will definitely help.