Yes mallard ducks will mate with Pekin's. I raised duck's the last 5 year's.My flock consisted of domesticated mallard's,pekin,runner, call and mixed standard duck's.These breeds are too heavy and fat to get any real height in flying if even off the ground.If you want to have purebred duck's you need to keep your ground duck's such as runner, pekin, Muscovy or the like mix in a separate pen.Most Mallard's domesticated or not can fly along with call duck's.So keep this in mind if you have open top pen's where these flighty one's may be able to fly in.If you want they will mix and they can produce some very nice color variation's and make wonderful pond or yard duck's.