Yep, I agree with the above posters. I have six huge eight week old Pekins in a coop with chickens and they do just fine. They love their outside time, eating plants and bugs, and swimming in the kiddie pool, but for bedtime, they follow the chickens in.
Getting ready to move them, however, not their fault, but the CHICKENS are apparently feeling crowded and decided to peck one of their roommates nearly to death last night. I am heartbroken today, as I had to put it down and I've never ever killed a living thing. Couldn't let it die that way though.
Here's a tip though, instead of worrying about a huge pool for swimming....you can get two cheap plastic pools at your local dollar store...they are easy to flip over for cleaning and refilling, and the ducks can hop in and out with no ramp. Also, if you are in a hot climate, ducks LOVE frozen veggies, mine love peas especially. I call them "duckie Popsicles"....lol.