Congratulations!!!
For me, I'm cautious about size difference with ducks. Depending on your setup and on the personalities of the ducks, you might be fine with a small breed like mallards along with your Muscovies, especially if the Muscovies are "in love" and the drake has no interest in smaller females.
However, there have been occasions of females being torn up by larger males either because of size difference or aggressiveness.
So you might want to bear that in mind when adding to your flock.

For me, I'm cautious about size difference with ducks. Depending on your setup and on the personalities of the ducks, you might be fine with a small breed like mallards along with your Muscovies, especially if the Muscovies are "in love" and the drake has no interest in smaller females.
However, there have been occasions of females being torn up by larger males either because of size difference or aggressiveness.
So you might want to bear that in mind when adding to your flock.