I'd give it a go, just make sure you stay out there and watch them for awhile. I have 7 grown hens, 6-4 week olds, 3-5 week olds, and a broody with 6-3 day olds, no issues. Let us know how it goes.
How old are the other pullets already in the coop? If there is only a couple week difference you may just be able to put them all together, supervised for a bit to be sure there are no issues.