I feel like any question on quarantine is summed up here:
Thank you,
@NatJ. Seeing you had quoted me instantly made me think I was in trouble for not being too worried about quarantine.

That did cross my mind, so thank you for bringing it up, and the OP brought that up too. Very good point.
I would expect he'd immediately try winning them over by courting them. He'll like put his wing down and do the dance. I don't know as he'd immediately mount them, but that will come with the first willing (or unsuspecting) hen. Roosters are exempt from the the hens' pecking order. When adding a new rooster, there is a chance they'll fight, especially because he's young and inexperienced. If there is any fighting, he is old enough that I don't think it'll last long and they'll eventually give in to him. (You'll probably notice some hens falling for him more than others.)
When's too much is when there's blood and it's going on for days and days. If there's direct bullying, then you should intervene. The best method of adding any new bird (that I've found) is add them at night. It minimizes the integration time by nearly a week, though they have seen him, so it probably doesn't matter at this point. If this was my flock, I'd just let the hens out to free range (supervised), then lead them back in when all is good (or when I'm going back inside). If all is good, he should go right in with them, though he might be again, nervous of the new place.
The ratio... is a little concerning. For a rooster his size, I'd actually say 18 hens, though you could still go with the typical 12. I have known people to keep roosters with that ratio, and I've even had a flock with a rooster with four hens. The biggest thing to be worried about is their backs and over mating. When you have more hens, that activity would be spread out further. (My small flock actually did just fine. But then again, my rooster was a different breed and was more respectful. There's no telling how he's going to be.)