congrats.
The probe for the thermostat should be in between your uth and the bottom of your enclosure (on the outside) no wire should be leading in and out of it. The probe for the thermometer should be attached under the substrate to the bottom of the enclosure (i would suggest a hot glue gun, no tape on the inside your snake will get caught in it.) Then adjust your thermostat accordingly the get the desired temp on the inside of the enclosure.

Yes juveniles to tend to be nippy. They are not used to people yet and see you as a threat. It just takes time for them to become used to you. But a bite from a rat is much more painful then one from a bp.