The rouen is basically a big mallard. Same colouration - different body type. The rouen I believe comes from France where hundreds of years ago selective breeding from mallards ended up in the super sized version of the original.
The thing is, being of similar color and marking that the genetics are blurred as the 'select' birds get mixed with the half-breed of the other side and you see something in between most of the time.
My little mallard hen is palm sized and full grown.
The "rouen" drake thats just a couple months older is 2 to 3 times her size....but he is a little smaller than a true rouen male from what I can tell.
The little one flies like a sparrow....buddy (drake) also flies but its not as easy. he has flown across the property to come to me, and it was quite impressive to see that big bird flying that fast..toward me. Not as graceful as the mallard, he is somewhere between a rouen and mallard I believe.
I guess bottom line is size which is the determining factor for flight ability. Smaller the bird the easier it is to fly and maintain control of flight. Buddy flies like a football while Chicky flies more like a bird, as we see in the wild.
A Rouen has a very heavy keel that nearly drags on the ground, making a Rouen box shaped. A Mallard doesn't have a keel.
The "Rouens" from hatcheries are some sort of gray colored mongrel, so no telling what you might get.
Just because two breeds are the same color does not make them similar in any way. Runners also come in gray color and they are nothing like Mallards or Rouens and Mallards and Rouens are nothing like each other. Lots of mutts come in gray color and that doesn't make them anything but a gray colored mutt.