When your new chicks get older and able to go outside, I recommend you place them in an area close to your older chickens so that they can see each other/get used to each other,yet seperated. Then at about the 2 week mark, put your newbies in the chicken house in the middle of the night with your older chickens. This will be less stressful for them. When they get up and leave the chicken house the next morning,the older ones will believe everything is status quo and go about their business. There may be some squabbling though,setting the pecking order straight. You'll have to monitor them for a day or two before they work it out.