It should be fine. I'd give them a heat lamp for a few days to ease the transition, if you possibly can, or put them out when there's supposed to be a few days of warmer weather. Just rushed to get my 5 week olds out to their new pen this weekend, so they'd have this spate of warm weather to acclimate.
Rocks and Australorps are notoriously hardy
I've never kept EEs, so I don't have any first-hand experience with them! On the size issue, I know my Aussies grew faster than my rocks did, but it wasn't that huge a difference. Can the rocks get to the feed okay, or are the others keeping them at bay?
Rocks and Australorps are notoriously hardy
