I recently began the 42-Leg Specialization from the last issue of NNM. What and how much cardio should I be doing to still allow for enough rest for my legs?

I would keep cardio to a minimum, if you do any at all. Maximum effort followed by ample recovery time is the key here. If you do anything, I would walk outside or on a treadmill for 30 min or less. Days 1 and/or 4 would probably be the best time for this.
CR