Honestly, yes, that is a bit soon. Although chickens can't count so don't know exactly when 21 days have passed, they have enough sense of the passing of time, that 7-8 days broody isn't long enough to fool her that they are her chicks. All is not lost though. I once had someone come and buy some 2 1/2 week old chicks from me to put under a broody. I was dubious it would work since at that age they had never had a mother and were also big enough I thought the broody hen wouldn't be fooled. However the woman insisted on trying, so took them home, waited until night fell, and then put the chicks under her hen. Next morning Mama Hen was proud as punch of her new brood and happily paraded them around the yard. The chicks meanwhile, took to having a mother like they'd never been without one. So perhaps you could get the chicks tomorrow and keep them in a brooder for 10 days or so and then give them to her AT NIGHT, and see if she will accept them?
As for your other question, Mama will try to protect them from the other chickens. I've seen lots of stories on here of chicks being killed by other flock members though, so you want to be careful. I have a Mama Hen right now whose chicks are 2 1/2 weeks old. I had her segregated in a broody coop for the duration of her sitting, and for the first two weeks after the chicks hatched. The flock could see and hear them and vice versa, but that was all. Earlier this week I started letting them out for supervised time out of the broody coop, and Mama Hen is doing great at keeping them safe. One thing I have noticed is that if a pullet who is lower in the pecking order than Mama gets too close, she will attack and rip their feathers out, screaming at them to stay away from her babies. But if a hen who is higher in the pecking order than Mama comes close, she doesn't even react. Fortunately, the older hens have had no interest in killing her chicks, and in fact seem very tolerant of the little ones.
