There might be a bit of a kerfuful when you first put him with your girls. Mine have always been in love by morning. Unless it is bloody, do not interfere. They work it out best without humans.
Personally, I think you are getting the best type of rooster, raised in a multi-geneartional flock, and so nice, he didn't get the ax, but do know not all roosters work out, and some are nightmares.
Do know, that the rooster will change your relationship with your hens, they become his. They will look to him.
I am assuming your hens are laying? If not, wait until they are to add the rooster.
Take a good look at the place you get him, ask it they have recently added new birds, or if they go to shows or auctions...all of those places would be a good chance of picking up diseases. If so, the quarantine seriously.
The quarantine issue is real, if you have a flock you would be devastated to loose, or if you have a lot of money invested in your flock, or you have a rare and valuable flock, you need to take quarantine VERY SERiosly.
But if you just have a flock, and this person has a flock, and things look pretty healthy, well you can risk just adding him after a day or two.
Thing is if you cheat at quarantine, you may as well not do it. Most people cannot properly quarantine in a traditional back yard set up, there is not enogh space.
Mrs K