How long before chicks go outside

We have two rhode island reds and two barred rock chicks that are around three weeks old. We have them in a brooder in the hottest room in our house and just close the vents so they won't get cold and they seem perfectly comfortable. We have nevee used a heat lamp, just because it got way too hot when i tried it and the room stays pretty warm. They are very playful now and they seem to be getting very bored in the brooder. I let them out during the day when i can be outside so they can explore. They flap their wings and run around and chase bugs and fly up to perch on things. We live in South Carolina and during the day it has been in the 90's and at night its normally in the high 70's and low 80's. I was just wondering when it would be okay to have them start sleeping and staying outside in our smaller chicken pen. They have their wing feathers and tail feathers and all, but they still have just down on top of their heads and down the middle of their backs. Also, how should I go about introducing them to our other four hens and rooster. Our rooster has come up to them before while they are out and sometimes he will not pay them any attention and other times he will chase them and try to get them. Our hens and rooster have a large coop with a very large run, so there should not be any overcrowding once we start mingling them.