It depends on what you mean by isolating them. I've always divided my run lengthways with netting and provided temporary roosting accomodation for the new ones, (ie a box with woodshavings in, placed under cover.) but this has always been because I've got them quite young, as growers or 16-week-old POL, and because I wanted to keep them on growers food and give them a chance to get to settle in and get to know the others through the mesh for a week or two. So they haven't been entirely isolated in a separate run, but have had their share of the space which will be available to them when the partition is removed. The mesh has to be quite high, or the hens tend to fly over it, in one direction or the other! But now I've sorted it out, I have a kit in store of netting and bamboo canes, the right size for my run, ready to use whenever I need to section off part of the run, which is useful.
As to 'quarantine,' i think again this depends on several factors - if I'd bought them from a sale or a market, where they'd had a lot of stress and possibly come into contact with dirty pens or other poor-quality birds, i would feel I needed to be sure they were OK and would quarantine them for this reason. But since I get mine from a known and trusted supplier of home-bred poultry, and they've been vaccinated, I have no concerns about their health and wouldn't consider this very important in itself
I don't think you know until you try, whether the 'friendly' older hens will take to new ones easily or not - when i got my two 16-week-olds this Spring, the worst bully was the previously bottom-of-the-heap meek-as-milk Speckledy, who was truly terrifying, so much so that I had to rehome her! But the two new ones were white hens, introduced to a pen with darker ones, and I have read that a big colour difference can cause problems when integrating newbies. But if you do all the right things, and your present hens aren't too old and set in their ways, it should go OK.