When I have new ducklings I am introducing to the flock, I do it over several weeks. My ducks who are new to having additions are usually the more aggressive. I take them out in a fenced area where everyone can see each other. But none of my ducks can get to the ducklings, a lot like the pen you have. By the time they are ready to go out, they have seen each other on the other side of the fence for a long time. When I do start to introduce them face to face, I let them share to the area during the day and at night they are seperate in the coop. I tend to use a dog crate in the coop for separation. Lastly they share the coop at night too.
With drakes, every drake is different. Mine has never cared about younger females and I can put them with him. Other drakes will definitely try and mate younger females. The Drake can injure and potentially drown and kill the younger female. Size is the issue. This is also the potential problem with too many drakes. You can get your drakes overeating females and gang up on one in the water and drown her. I have had 13 females and 2 drakes at one point. I had to rehome one drake because he was too much for my flock and causing my other drake to stress out. He went from a very sweet drake to his girls to getting crazy eyes. After removing that drake, the flock calmed down and the my boy became sweet again for a drake. Mine didn't fight, but some do have that issue.