If you do a raised coop in the pen, odds are the ducks will nest beneath the coop. If you do a coop on the ground, then the duck might nest with the chickens
Wherever their water is, it's just going to be messy. That's why I don't keep them currently. So it would be easiest to just designate a corner of the pen as the water corner and just keep the bowl there
They'll mKe the pen miserably muddy for chickens. And if there are drakes, absolutely not. They could seriously injure the hens if they try to breed (which they will).
If it's two duck hens, then yes, usually it should work