Hens have been raising chicks with the flock for thousands of years. They are living animals and anything can happen, but they have been successful enough raising them that way that they are not yet extinct.
There is no right answer or wrong answer to this. We all have different flocks, conditions, and set-ups. I really think how much room Mama has to work is important to the overall success, but if space is too crowded for her to raise the chicks with the flock, how are you going to integrate them later? You won't have room.
I prefer Mama to integrate the chicks herself, but I have plenty of room. I integrate incubator hatched chicks a lot without problems. But some people have a history of problems so keeping them separate works for them.
As far as the feed, there are plenty of studies that clearly show eating Layer can be bad for the growing chicks. I'm not talking about people that once saw a chick eat a bite of Layer feed and did not drop dead immediately so all that is a load of bunk but studies where Layer was regularly fed to the chicks and medical professionals cut the chicks open and saw what damage was done to the internal organs.
The problem is the extra calcium in layer. One bite will not kill them. They need to eat it regularly for a few days to suffer harm. And it is not about the percent calcium in the feed that counts. It’s about the total amount of calcium they eat. If they are foraging for a lot of their food and not eating much Layer they are not likely to be harmed.
Broodies will take feed out of a high feeder and drop it on the ground for their chicks. By the time they are two weeks old, those chicks will be flying up there to eat it by themselves.
What I suggest is feeding the entire flock what you want the chicks to eat and offer oyster shell on the side. The ones that need the extra calcium from the eggs should eat enough oyster shell to get what they need. The ones that don’t need it may experiment a bit with it but shouldn’t eat enough to cause themselves any harm.
As others have mentioned the hen will keep them warm. Her heater never has a power outage. Winter time is not the best time for a hen to raise chicks, but many do it successfully.
I don’t know enough about what your set-up looks like but if you have space I’d suggest let Mama handle it in the time honored way. I can’t give you any guarantees that nothing bad will happen, but no one can honestly give you any guarantees that nothing bad will happen if you go a different route.
Good luck!!!
