You should not try to integerate them until they are at least 16 weeks old and near the size of the grown chickens. Otherwise they won't be mature enough and have enough size on them to defend themselves from the attacks associated with being the new chickens in the flock and the assertion of the new pecking order.
When you place them in the main flock, they should be in a pen where they can be seen and limited contact with the other chickens so that they can get used to seeing them. This usually is about 2 weeks for me. By the time the new chickens are 18 - 20 weeks of age they are getting near the point of lay. You can continue to put out their starter/grower feed. Your older chickens will eat the starter feed and the younger chickens will eat the layer feed. They will be fine. When the younger chickens start to lay and you run out of the starter/grower then give them all the layer feed.