Here's a calculator to figure protein content if you mix in oats or other grains. There's even a list of grains at the bottom:
https://www.metzerfarms.com/FeedConversion.cfm?affiliate=undef&CustID=29088871
Pekins are meant for meat production, and they do grow fast and huge. You should try to limit the fast weight gain, whether that's with lower protein, more veggies, exercise, etc. because they can get so big that they have awful joint and mobility issues later in life if they're not raised for meat. Mine love a bag of frozen peas with water in a water pan during the summer, frankencucumbers that were pollinated by the pumpkins, and of course, pumpkins. My ducklings are insane for watermelon, the adults, not so much.
Since they're all over a year old, and the babies are almost 6 months old, you should be able to cut back to about 16% protein without any problems. Your layers need about 17-18% protein while laying and free choice calcium. You could accomplish this by providing insect or dried fish treats (some bugs have more fat than others, so watch for that). They need about 20% protein during molt.
I start my ducks on Purina Duck food + 30% game bird starter to get the protein up to 22% and cut out the game bird after 4-6 weeks, I start chicks on Nutrena non-medicated, then move everyone to Nutrena Naturewise All-flock at around 18 weeks. Instead of scratch during the summer (it makes them hotter), I give them wheat feed (the feedstore calls it wheat berries, it's like $8 for a 50lb. bag). I'm trying a bag of the TSC brand (DuMor? Producer's Pride?) All-flock as it's a bit less expensive and still has good numbers and probiotics. They don't seem to gobble it up like the other feeds, but I moved everyone to a much larger yard about the time I started that bag, so it's probably because of all the grasshoppers and weeds they get to eat now.
