CO2 is the way to go for pre-killing a group of rat pups. Drowning and freezing will cause water retention issues.
If your local store can get F/T rat pups, what's wrong with continuing to feed the large mice until they come in? Honestly, we mostly likely overfeed these animals by keeping them on a weekly schedule anyway. It's not going to cause any damage to keep him on mice until the rats show up.