you can put them in with the hens when they are old enough to be without a heat lamp, then watch them, some hens will accept chicks and leave them be others won't, if they peck the chicks a couple of times that's just them establishing their pecking order, if they peck them run after them and attack them it's best to take them out and put them back in when they are bigger
The easiest and best way to introduce new chicks to adult hens is to brood the baby chicks in your run or coop where the adult hens will see them everyday as the chicks are growing. This establishes the chicks as members of the flock from the very beginning, and they aren't total strangers when you get ready to put them all together.
It also gives the baby chicks several weeks to learn about the different temperaments of the big hens, and they will know who is going to be nice and who is likely to be mean. I usually let my chicks start to mingle with the adult flock between ages two and three weeks, using a "panic room". This is an enclosure where the chicks can always run to for safety where the adults can't follow.
I go into greater detail about this system of introducing new chicks in my article about brooding outdoors. It's the second one linked below my post, and there are photos of the panic room system, too, to make it even clearer.