Very young chicks from reputable sources are unlikely to be a source of disease/pests, so as a general rule it's safe to have them around your flock without a quarantine period. It works both ways though - i.e. if your chicks are vaccinated against Marek's the vaccine needs some time to...
I'm another person on the "introduce 'em early" train. I started my last batch of chicks outside at 2 days, they started mingling with adults a little past 2 weeks, and moved into the coop before 6 weeks. Another bonus from early integration is the chicks learn to "chicken" from watching the...