Mallards and muscovies are quite different is size, so I'm going to guess that they wouldn't have success even if they tried to breed. But if they did, the offspring would be "mules", which means they will be sterile. That happens when a muscovy mates with any breed other than a muscovy.
Are you looking for a muscovy drake? I have a bunch for sale right now. I am just trying to find them good homes. I am located in southern Maine, you can PM me if you are interested.
I also have a bunch of female rouens. I know you said you have mallards, but rouens are very similar to mallards. They look almost the same, they are just larger. I am moving soon and need to rehome a bunch of my ducks. Unfortunately, the rouens are all going.