are pekin bigger than rouen?

[Ratchnick]

My understanding is the OP has hatchery birds not show or exhibition quality birds.

Right, but my point is you can likely still get production birds that weigh much more them 5 lbs

 

[21hens-incharge]

There is a big weight difference in different rouen strains. Exhibition rouens can be about 12 lbs.

https://www.feathersite.com/Poultry/Ducks/Rouens/BRKRouens.html

I doubt the OP has birds like this but it is something to look at.

do u think a rouen hen could still do good with a pekin drake? with a couple more hens added?

No I do not think it is a good idea. My suggestion is to get pekin females for your pekin male.