In my experience it's less of an issue (usually no issue at all) if the youngsters are with a broody, since she will guide and protect them. However, for chicks I've reared 'artificially', I'll keep them separate and on growers until they're POL.
I believe there are issues with the calcium and protein contents of the different feeds (I'm sure somebody more experienced will be along in a minute to advise!).
One thing I do know is that if I just leave a broody to get on with it as part of the main flock, if I put down chick crumbs she will not always guide her young to them. This means they often get eaten by the others before she gets the chance, and I'm sure you've seen the price of chick crumbs! Also although the chicks get there in the end and probably with a more natural diet, they definitely grow far slower than those reared in a shed with ad lib feed.
HTH!