From what I've learned here, hatcheries have to mark domestic mallards in some way to show they are domestic and not wild mallards. So when I was buying mine at TSC I told the employee I needed to see the back legs of them to make sure they were mallard and not rouen. I knew which hatchery...
They still look like mallard but time will tell. If they turn out to be gigantic then they are rouens. If they stay small, then mallards. Also might be able to tell if the hatchery they came from, removed a back toe.
Interesting. I thought mallards were one full and sometimes one broken stripe, and rouens were two full stripes. Learn something new every day I guess. They are adorable.