Feeding different age birds?

I'm getting new babies in this week. They'll be inside in the brooder till they're feathered enough to go outside. Once they move outside, the plan is to house them with the big girls, who are six months old and laying.

Big girls are on a seed-and-grain mix, which I make at home and is formulated for 17% protein, plus greens, grit, and oyster-shell free choice. Babies will be on DuMor starter. I'm fencing in the garden soon and by the end of September they'll all have half the yard (about 900ish sq ft to free-range and help prepare the garden area for next season's planting.

Are the nutritional needs for 8-week-olds and the nutritional needs for 8-month-olds different enough that I will need to provide two different types of feed, and if so, will there be any problems if the big girls eat the babies' food or the babies eat the big girls' food before they're ready? How do I keep them separate so they don't get into each others' food if they'll all be living in the same space?

You can mix the starter with the regular feed to increase it's protein content and keep the oyster shell separate. The chicks may peck at the oyster shell, but they won't eat enough to do harm. The only difference is that the growing chicks need a higher protein content and the laying birds need access to calcium. Introductions should be done very slowly. 6 month old pullets can kill 8 week old chicks, so don't just put them in the coop with the big girls and hope for the best. They should be kept separate, close enough to see each other and interact, but not touch for a few weeks. Then start supervised free ranging or run time together for a few hours at a time. Once everyone is mostly getting along, you can leave them alone together full time. Be sure to have multiple food and water stations so that the older girls can't keep the little ones away from the feed and water.

I'd planned on allowing the babies some daily, supervised outside time in a separate pen from the big girls starting when they're a few weeks old, in an effort to get the chickens used to each other before it's time to move the littles outside. There is space in the yard to add a second coop and run if necessary, but I hope they can all get along well enough to share sleeping quarters. The brooder is NOT predator-proof, so I would have to buy or build a second coop and run, and there's only one other place I can put it that's still within legal boundaries (it has to be ten feet from property lines). Current coop is the size of a small shed (6ft long, 6ft wide, 7ft tall) and has two 6-ft-long roosts (which my chickens *finally* use, even though they all still sleep as squished together as possible in a pile... weirdos) and two nest boxes (they all use the same one). I leave the coop door open all day right now so they have access to the nest boxes and shelter from rain or sun, but they spend most of their time out in the run.

Before the little girls are ready to go outside I'll have the whole garden fenced off and there should be *plenty* of space for seven birds to roam around. I'll be sure to put some "hiding spots" for the babies, too, along with extra food and water stations.

Also (and I forgot this before) in case it isn't clear, I'm getting the new ones as day-olds and they won't be moving out until 8ish weeks old, around the start of October. Right now my big girls are 6 months old, but by the time the babies are big enough to move the big girls will be 8 months old.

There are two differences in chicks needs and laying hens, protein and calcium. Since you already have the oyster shell separate the only thing to do is raise the protein level in the feed you mix.while the chicks finish growing.

Consider having a temporary coop in the garden for the chicks to use with a fenced off area for them to use to give them a chance to grow a little more and get to know the older birds for a couple of weeks.