I don't know how much you know about quarantine or why you might want to do that. Some people quarantine, some don't. If you wish I can give you my opinions on why and when you might want to quarantine but that's a lot of typing. I also do not put a lot of faith in a rooster being that much protection to a flock but obviously you do. Your opinion is the one that counts.
If you get a new rooster, get a mature one. You don't want a bratty adolescent that is controlled by his hormones and likely not respected by the hens. You never know how any individual will react or behave, but your odds of a smooth integration are much higher with a mature rooster, at least a year old, than with an immature cockerel.
I've never had a problem with a mature rooster and chicks, either chicks he sired or chicks from the post office. That's chicks given to a broody hen roaming with the flock or raised in my brooder which is in the coop. Some people have so you need to watch. Some people have had broody hens kill their chicks, about anything can happen. A good mature rooster should take care of every member of his flock. Some ignore adolescent boys and girls or may keep rowdy adolescent boys in line when they bother his hens but until they are seen as rivals I haven't had any problems with a rooster and chicks. He should assume any chicks are his. My roosters generally ignore any chicks but sometimes one helps Mamma take care of them.
I don't know how much room you have. Bad behaviors are often magnified when room is tight. I'd expect the broody hen to keep her chick away from the rooster and the rest of the flock most of the time if she has room. I'd still expect a mature rooster to not harm a chick, I'd expect the chick to be at more risk from you other hens from my experience. But the tighter the space the more I'd watch.
I would not let concerns over a new rooster killing a chick stop me from getting a new flock master if you really want one. Just get a mature one. I'd worry more about integrating a new hen.
