Since you are getting good advice about how to integrate, I will address the feed problem. You can keep the older chickens on grower food that way the youngsters won't be getting too much calcium. I feed mine Flock Raiser and just offer oyster shell free choice. I don't have a mixed flock yet (still waiting for my youngsters to get big enough to be in with the "big girls") but I wanted to increase the protien my flock was getting. The shells on my eggs are very hard. The youngsters won't eat the oyster shell until they need it. BTW I saw a couple "rules of thumb" about integrating younger with older. One was to wait until they are about the same size and the other was to wait until the "chicks" don't sound like chicks any more. I am waiting to fully integrate mine until they are at least about the same size.