Hmmm.. usually people don't introduce chicks until 6-8 weeks if possible. But if you think they are ready, then it's ok. It depends on a matter of things, the chicken's personalities, the breeds ect. But, since you have such a LARGE amount of small chicks compared to you older hens, I think it'd be ok. I'd suggest letting them out one morning where you're going to be home all day, let them all out in the morning into the fence area and watch them. I'd be surprised if the hens don't just about ignore the chicks, but just watch to make sure that nobody is attacking. Then at night, put them all together and see how it works out. I'd put them in, there is going to be some squawking, just let them work it out for about 10 minutes, then go outside and if it's still going on, then go ahead and intervene.
As for feed, a lot of people on here will tell you to not feed layer to your chicks because there is too much calcium. We used to do it without too many problems, but our hens also free ranged a lot so they basically only ate the feed when it rained. If they have plenty of room to free range they'll be plenty ok. But, if they don't have a super a lot to free range (it's not just area, but also what kind of things are available, is there a garden they can go through, or is it just grass? is it just dirt? ect) then I'd suggest getting a standard flock raiser then just sprinkling oyster shell on the ground, the chicks won't eat it if they don't need it.
