I got my chicks well before the ducks hatched. I tried putting the ducks in with the chicks day one, however, the chicks wanted to peck at them a bit, so I gave them their own 30 gal tote for the first week. Then when they were as big/bigger than the chicks I put them back together. The chicks got curious and went over, the ducks pecked at the chicks and the chicks jumped back. They have been doing fine ever since.
The ducks do go and run under the brooder lamp occasionally when the chickens are having their little pecking play fights, but they leave the ducks alone... And then they go cuddle with the ducks when they feel tired. :)
Now, I have taken one of the chicks out to the main pen and the teen chickens started pecking on her, so I built a swank little screened box in like 2 hours and put that in the pen so the teens can see the chicks and get used to them before they are all together. Same will go with the ducks, since they all live together.