Introducing 2 flocks

Hi! First time chicken mom (was waiting til we bought land and happened to be the year of the chicken boom apparently!). We got 4 little babies one month ago today. They live in our bathroom in a brooder right now.

Yesterday, I rescued 8 babies who range from 2 weeks younger to 1 week older than mine (based on appearances). They were not cared for very well, just kind of left in a lackluster coop outside until they changed their minds. I have brought them back inside in a second brooder as I’m not comfortable putting them back outside just yet.

My questions: how long do I keep them quarantined for health purposes?

What is the best way to integrate 2 small flocks of the same age when nobody has lived in the coop/run yet?