I recently added some small chicks with a pair of older broiler chickens and had no problems at all, aside from from establishment of pecking order, a couple of days they were accepted fine. My experience may be the exception to the rule - the Poultry Board would be a good place to ask about this.
What do you mean by "turn loose"?? Are you free ranging?? I have "turned loose" chicks of the size you are speaking of in a free range situation without difficulty, but ran into problems with penned birds. What I have done since then is have sort of a "creep feeder" situation where the little chicks could get in but the bigger ones could not and they slowly got use to each other without problems. I would be most concerned about the young roosters if you were planning to grow them out for meat. They spend a great deal of time watching out of the senior rooster who wants them to just plain not be :no:
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