In our flock the roosters leave the young ones alone. We put our babies with the flock at around two months, but they are with their mother usually, although sometimes just their siblings. The roosters seem to know that the pullets are too young to do anything with. You should quarantine any new arrivals for a month or so anyway, then you could try putting the young ones in a pen where they can see and socialize with the rooster through the wire, and just see how they interact for a while. Then you might have a better idea. When ours are quarantined they are in a pen where they can at least hear the others, and vice versa.