Can I put my 6-week-old Silkies outside overnight with no light?

I have 14 chicks in a brooder inside. 11 are 7-8 weeks old and fully feathered. I'm not worried about them. The other three are 6-week-old Silkies, who seem to be feathering slower than the older ones did. They still have fuzz on their backs or bare backs, though their wing feathers are fully in. I'd really like to put them all outside in the coop together since they get along so well with the Silkies. It still gets down below 40 occasionally here over night, though most often is warmer than that. They will be in an 8x10 coop with our 9 slightly older ones (though separated in their own space for now). They've been off lights for a while inside now, and the room they're in sits around 65 degrees. Will being with so many other chicks provide enough warmth or should I wait another week or so?