How old were the pullets when you got them? I take it you have no experience with chicks at all.
Basically, by the time chicks reach the age of the ones you are planning on buying off CL, cold and heat requirements are no longer issues. By age 10 weeks, chicks are around 85% of their ultimate size, nearly grown. However, they will have an instinctive fear of adult chickens. Your two adult hens will pick up on this and exploit it.
Therefore, your concerns will be to provide safety and security for the new chicks while they integrate into the flock.
@AlvarezAcres has mentioned how they handle it. The objective is to get everyone used to one another without the newcomers getting physically thrashed and their self confidence ruined. At the very worst, they may be driven from food and water and will suffer nutritional harm.
I wrote an article for BYC about how I integrate new chickens into my flock. It's linked below. After the first night everyone sleeps together in the coop, while they have a safe pen during the day. For juveniles still smaller than the adults, you will need to establish a safe feeding and watering station for them after the initial integration. I use a card table in the run for this purpose. Elevating these essentials gives the newcomers a resting place the adults aren't interested in.
As far as quarantine goes, it does nothing to prevent the spread of serious avian viruses. Chicks and adults can be carriers yet show no symptoms. Far better than quarantine is to make sure the poultry you plan to buy comes from a NPIP certified source. However, buying off Craig's List opens up the possibility that the chicks have been exposed to one of these viruses since the time they came from the original source. Not intending to scare you, but you should be aware of the inherent danger.