I think Randy's advice about right sizing space for the bees is good so I would definitely consider moving them into a nuc box. How many frames of bees are they? If they are only 1-2 I would also suggest adding bees from one of your other hives to give them a boost into being larger going into the winter. There is still plenty of time for them to outgrow a nuc box. Shake a couple of frames of bees into a plastic tub, wait a couple of minutes for the flying bees to leave, make sure the queen isn't in there and then pour them into the small hive. Some of them will still go back to the main hive but some of them will stay.
I also like the idea of overwintering some of smaller hives in double nucs but I am worried about possible toppling over so I have to think about it.