I am very sorry about the loss of your little duck.
Disclaimer: My experience with ducks is limited to runners; I've never had Pekins. But, I agree with @Jenbirdee about keeping the older drake away from the youngsters. Even my mild-mannered runner drakes go crazy with hormones, especially in the spring, and get far too excited about "romancing" the girls.
For MOST of the year, my two drakes hang out together without incident. But mating competition overtook last month, and the younger one sustained a wing injury in a squabble with the other drake. The wing, which hung limply at his side, luckily wasn't broken. But the infection went down to the bone and has meant a month's worth of twice-daily antibiotics.
Best wishes with integrating everyone, and welcome to BYC. Let us know how things progress, please.