Are your female ducks that are in with your ganders Pekin? It will most likely be okay for them to breed. My gander [Embden] mated with all the Muscovy females here for 5 yrs until I brought home a female goose for him. You do have to watch them when in the water though. Even now My Runners will get in the pool with my gander and I make him get out he is way too big for them. Chinese being smaller and weighing less shouldn't be a problem one on one. It's when they gang up things can get nasty.
Try letting them live side by side for a few weeks then put the female ducks in with your 2 ducks who have been recovering see how that goes if things go good after a few days try introducing the ganders into the mix. That maybe trickier.