You should be totally fine adding a few more. There will be a little fighting initially while they figure out who's boss, but since you are wanting to add girls, things should settle down pretty quick.
I've had a pekin drake with rouens and khaki's. They did fine, I don't think you'll have a problem with the size difference. Rouens actually (at least mine) are only slightly smaller than the pekins. Now if you had a muscovy drake, I would be a bit wary, I had one accidentally kill a khaki trying to mate her. Those guys are beasts though, way heavier than my pekins.
I would personally add a couple of the same ones if your going with the smaller ones like khaki or runners, then Mr. Pekin doesn't lavish all his attention on one small girl. I always had a least a few girls of similar sizes.
Also, yours sound a lot like pets, I will warn you, runners are pretty flighty! I love mine, and they are very fun to watch. But no matter how much time I spend with them when young, they run from me like i'm the devil. Even when I bring food, they wait till I leave or have my back turned to indulge. If I have to herd them of move them, they literally trip over each other in a dash to get away. All the other ducks come right over for food, but not the runners. Just thought I'd mention that!