Hi Hilary, and a big welcome to the Forum!
I wonder how big your run is, (bearing in mind that the recommended run space per chicken is 2 sq. metres, certainly no less than 1 sq.metre.) If you take on 40% more chickens and they don't have enough space, it would be difficult to integrate them with your existing flock and they would be likely to resent newcomers and fight. You might well get ongoing problems later, with feather-pecking as well if they're overcrowded. As your group are all adults this process might be a bit tricky anyway, but it could be managed and there's lots of advice on here about how to go about it. As you'd be adding quite a few, the bantam would be less 'exposed' within her own known group of hens, so if she's not at present being bullied by them she should be OK.
You're right to think that the new ones should be kept separate for a couple of weeks, but this is mainly for quarantine reasons - you don't want somebody elses's chickens possibly infecting yours or carrying parasites, especially as it sounds as if neither the previous nor the present owner was/is experienced with or greatly concerned about chicken welfare. Any change of living conditions is potentially difficult for chickens and often they do go down to health problems soon after being moved because stress lowers their immune system's resistance. Does the extra house and run come with the chickens? If so, do be careful, it may well be infested with redmite, which you don't want to import!
If you're concerned about the welfare of these chickens in your neighbour's inexperienced hands, maybe you could offer to help with looking after them for a while in her garden, and then try to rehome them for her? You could advertise them on the For Sale section as Free to Good Home.
Of course it all depends on whether you a) want more chickens anyway and have room and time for what would be quite a large number
and b) whether you want these particular chickens, having inspected them to see what you'd be taking on.
Do keep in touch and let us know what you decide, this sounds an interesting situation!