for storage, try to stay below -20 degrees celsius or -4 degrees fahrenheit. if you are just a bit below freezing, worst case is that bacteria in the guts of the mice or rats get active, and produce enough heat to keep the belly of the mice/rats above freezing temperature for a while. Then they basically internally rot in the freezer. Freezers of the crate design, a box with a lid on top, tend to be most energy efficient for this.