If the older chicks will be 8 weeks by then and it's next to the big coop, I'd go the big coop for all of them. And put the new mama and babies in the tractor
I've found early integration is easier. Raise my babies in the coop with the help of the whole flock. If you feel that's not possible, then another option is moving current mama and 5 week old babies into the coop (line of sight blockers without places to be cornered are a good thing here) and...