I've never hatched chicks under a broody, always in an incubator - so no Mum. I've never tried to put anything younger than POL in with the laying flock - once they start laying, I integrate them. Actually, I start earlier than that - the Marans are 6 weeks old and they and the laying flock get their corn together - except the chicks are in an enclosed run. So they get to see each other twice a day and the big girls forage around the cheeps' run. We'll do this until they start to lay, then one night we'll sneak the youngsters into the big ark and hope no-one notices. So far, it's worked fine - although ours are free range, so if there is any bullying, the victim can leg it out of range.