How old are they?
Around 5 months or so (some earlier, some later), the males develop a drake feather. Before that, voice sexing is your best bet. If it is a real quack, it is a female, if it is a raspy/hoarse sounding attempt at a quack, it is a male.
If the pekins and mallards weren't brooded together, then you have to take time to introduce them. Ducks don't like change and that includes bringing in new friends. Some may not care, but others will chase and grab the new ones if given the chance.
I have 4 ducks, all girls. 3 of them do not like the new chicks that have been introduced, the fourth doesn't even care.
Take introductions slow. If they are all out and about, supervise and don't allow chasing of the new ones. If they are penned, the new ones should have a space of their own for a while where everybody can see each other, but not get to each other that way they can get to know the new neighbors through the fence. Usually once everybody is the same size, you can let them fight their own battles, but with old pekins and younger mallards, you are going to have a good size difference, so you will want to keep an eye on their non fence interactions for a while.