Thank you, that is helpful infoSome roosters will recognize that pullets are too young for mating and will leave them alone, and some will not. You won't know about yours until you introduce him and watch what happens.
If he does bother the pullets by trying to mate with them while they are too young, you will need to separate him again until they grow up. You want him able to interact with the pullets but not hurt them, so a wire mesh divider or a wire crate can work really well.
Once the pullets start getting red combs and squatting, they should be old enough to mate.