If your pullets are close to the size of the hens, then I would go ahead and add them in. I have had the best results by adding them in a group in the late evening when the rest are all ready starting to roost and the light is low. I have also added a small group by first putting them in a dog crate and leaving them in the coop for a half hour first, and then letting them go. There is always a bit of a fuss, but I have found that as long as I have a couple of water stations and a couple of feed stations, AND the youngers are close to the size of the adult hens, then they eventually settle and do ok.