The sooner the better, but you need things set up correctly in the coop/run when you do it, and you need to have enough space. Many of the posters are finding that the sooner you introduce them, the less time it takes. However, the big girls can be very aggressive in the beginning.
Chicks need a safety zone, a place where the big girls can see them, but can't get to them. You could bring them in at night if you want for a couple of days.
Wednesday I went and got day old chicks, and the broody hen has had them outside by Thursday. They are sturdier than you think.
However, in my set up, I have shade, I have platforms, I have an outdoor shelter, I have multiple roosts, and I have a 20 x 30 run.
Take a picture of your set up, measure it up so we know how big it is. Very difficult to give advice as we seem to picture the question in our own set up, giving advice that may be wrong in a different set up.
Mrs K