The optimal age to integrate is 16 weeks of age. You can do it a little before then but there is always the risk of injury or death, depending upon the personalities of the older ones.
You want them to be about the same size as the older hens before integration, ideally.
Temp. requirements for chicks:
90-95 degrees the first week of life and decrease by 5 degrees per week until fully feathered even on the head, usually at 6 weeks of age.
They need space to get away from the light, to avoid overheating as well (a cooler spot in the brooder).
Unfortunately you are looking at a long time in the brooder for the chicks, and a lot of time separated from the older hens.
I would recommend you get/build a little tiny coop/run, as it always comes in handy anyways, as a growout coop. It can be used for sick/injured birds as well as if you ever want your hens to hatch out some eggs.