This is very true!
Your "rooster" is actually a cockerel (a male bird under a year old) and that makes a big difference behaviour wise. He is an adolescent at the whim of his hormones and without any adult birds to keep him in line and teach him some manners he will run amok with your pullets. It is not unheard of for pullets to scalp themselves trying to escape their attentions and they can also sustain gash injuries down their sides under their wings as well as the stress putting them at risk of other health issues like an outbreak of Marek's disease.
This is a human contrived situation due to incubator reared chicks growing up without older flock members. Yes it is "normal" for the cockerel to do this, but in a more natural flock dynamic with adult hens and a dominant mature rooster, he would not be allowed to do so or at least his opportunities to do so would be greatly reduced and the pullets would be able to stick close to a senior flock member for protection.
In my opinion, this is a man made problem and unfortunately there is not an easy solution. If you pen him separately, yes, he will probably be stressed but which is better, one bird being stressed or most of them as well as the risk of injury. Most of us get chickens for the eggs they produce, in which case the welfare of the pullets is surely our primary responsibility.
In case you have not seen a flock master rooster at work, he gains the respect of his flock by finding tasty treats for them and calling them over for it, he escorts them about, checks out suitable nest sites for them and encourages them to lay in them and oversees them. He wing dances to woo them and when they accept and squat, he jumps on them and holds onto the back of their neck to balance for a few seconds whilst the hen lifts her tail and he deposits his sperm. It is over in a few seconds without trauma or squawking. This is the way it should be, but such a mature rooster was once also young and at the uncontrollable whim of his hormones too. They mostly get over this horrible adolescent stage but many do not have the attributes to be a good flock master even then.
