I agree with speckledhen. Also the chicks will need a heat source where the 8 week old ones should be pretty well feathered.
Here is what I done. I kept my new baby chicks in the house (of course) for 3 weeks and then took them out in a cage that had a baby waterer and feeder in it and let the others get use to them being in there with them. Then when they were pretty close to the same size I let them out with the others. I stayed out there with them for over an hour to make sure that they were not attacked or hurt. There will be a little pecking order going on but that should be all. If you dont have a cage for them you can always make a cage for the babys that is made from a couple of 4 x 4's and some chicken wire. Oh, i forgot to tell you that I did let the babies out for a minute of two each day until I let them out for good. Also, dont forget their heat lamp. I just made sure that the babies didnt get to hot. I kept a thermometor in there with then.
