I've had very good luck doing what you're talking about. since your rooster is mature, and has access to breedable hens, he's not going to be a sex starved, crazy young maniac trying to breed anything remotely female. He
should be a gentleman, and wait until the pullets are approaching maturity to breed them. That's what mine have done, anyway. I would keep an eye on him, to see how he acts, but he should be okay with them.
If he's not, if he's trying to forcefully mate pullets under say 4 months, I'd look at getting rid of him, myself. But that's your call, you could keep them separate until they're of age.
Now, if you're putting immature pullets in with another immature cockerel, or an older male who has no access to a breedable hen, he may try to mate them out of frustration for not having another outlet. I may not cull a rooster for that, but I'd still keep the pullets separate for their protection.