momto7, Welcome to BYC! No it is a bad idea to place them together without a formal introduction. This is done by placing the smaller ones in their own smaller pen inside the larger one, for the others to see them and get use to them without the ability to hurt them. Maybe section a area off for them with their own feed and water, or whatever works best you. Pecking order is real in the chicken world and the young ones aren't big enough to take on the larger birds. Once they are 18 to 20 weeks old it won't be a problem. If you can take the time to police a attemp to put them altogether, that is a option, however not one I would recommend. Good Luck and welcome to the wonderful world of chickens.
bigzio