It takes time for ducks to accept each other they look at the younger ones as intruders and should be run off. I’d have the younger live side by side with the older ones till they can be out together with out anything but a little pushing or shoving but it doesn’t last an they go about their business. It takes time but they will be one flock. As for feed when we say 5-6 weeks for food 24/7 that is not for when they go outside it’s for while they are in the brooder. It use to be an still is for many that 5-6 weeks until fully feathered they are inside in a brooder. I still leave food out during daylight for my flock but they go with out over night.