1. How do you sex the babies?
The males at birth have a larger "knob" and it's further away from the "pooper"
I sex babies the day their born.
Flip em over, pull the loose skin on the back to tighten up the skin on the belly and the organs will stand out better.

2. How many females can I keep together in a 10 gallon tank during recovery?
None, 10 gallon tanks are for fish, rats don't do well in glass tanks.

3. How many rats can I keep in a 10 gal grow out cage.
Same as #2, wire cages or racks built for rats are a much better way of keeping any of the rats, hold over or not.
If your hold overs get myco being in a glass tank and then it spreads to your breeders....no fun