"What is the best way to monitor a hand fired unit and keep from overfiring it?"
A baro and don't forget to close the ash pit door. Forgeting the door is the single largest problem relating to overfiring.
"I have a magnetic temp monitor on stack."
Keep it under 750-800*
. If your stovepipe is glowing orange or even yellow (it will disappear at the next color), back it down as you are overfiring it.
"Should I install a probe of some kind inside the fire box as well?
If so what is a good temp probe for fire box temps?
I have no problem if I need to drill and tap exterior of stove to get probe installed."
No, if it were needed the 40 or so manufacturers would offer it.
Follow safe practices and you will never have a problem overfiring your appliance.