Ok, the leg issue may be due to them lacking niacin in their diet. The feed you are feeding isn't adequate enough, and the yeast looks to be livestock grade, so the niacin content per scoop is going to be somewhere around 2mg most likely. What I'd suggest is switching to a feed formulated for all poultry, or one specifically for ducks. Purina Flock Raiser, All Flock Feed, Mazuri, and Manna Pro Duck starter are a few good brands.
*Most* of these brands tend to have a niacin content somewhere around 55/65mg per kilo, and that'd be fine for your medium/smaller weight breeds, but with Heavyweight Pekins we really want to aim around the 70mg/kg mark, so an addition of nutritional yeast (Which contains more niacin then brewers yeast) or niacin capsules into their water would be advised.
Offer that to them free choice, give them plenty space, provide deep water, and you'll have healthy ducklings.
You can stop the niacin supplement after they reach eight/ten weeks of age.