I have no problem with combining 8-week old chicks with an adult rooster. Minimal pecking can expected initially as myhubbycallsmechickeemama relates but advantages can be realized where rooster provides leadership in respect to roosting location amoung other things.
Under natural conditions with red jungle fowl, that is how new family groups / harems are formed. A rooster without a harem begins associating and roosting with juveniles from other groups that have either been dispersed by predator or like or have been driven out by adults.
I am trying to see if that normal social rearrangment can be used as a management tool for free range flocks on my little patch.
Generally speaking, adult roosters are much more accomodating than hens when it comes to juveniles.