I truly think you have a male and female....If they were both males, both would have the green on the head. But I am probably the last person you would want trying to help you sex a duck. LOL. I have 3 rescues myself and one purchased drake. He was supposed to be a Mallard as well, but I...
If these babies were "rescued" from the "wild" then the chances of them being Rouens are nil to none. About the only way that a Rouen would be out in the wild is if he escaped from a farm. They are not flyers, so if one did escape a farm, the chances of surviving in the wild are very little...
Rouens aren't listed because they are not a "wild" waterfowl. They are generally a farmed bird for meat...thus why they do not fly like a mallard. They are a heavier bird usually produced for meat.
The website I posted has pictures of the feathers of each sex, and maturity that you will need in order to determine the breed. Also, if you dig a little bit, the website also has lots of other species of waterfowl listed. They still look like Mallard to me. But that's just my opinion.
I was trying to find some help in sexing two rescues that I have and I ran across this website: http://www.npwrc.usgs.gov/resource/birds/duckplum/mallard.htm
It will help you identify the ducks you have. You also need to keep in mind that male ducks of any wild species will be brightly colored...