leaving live rat pup overnight........good or bad idea?

Oh...I was talking about for a hatchling. I didn't know how big this guy's snake actually was...but figured if he was gonna leave anything in there that couldn't hurt a snake, a pinkie or two would be a good alternative

Unless you get one of those 18 gram twins, a hatchling ball python isn't going to be eating pinkie mice. I only try those on rare occasion when I have a stubborn starter, and nothing else has worked. I've never had that work yet, either (but they do seem to like mouse fuzzies sometimes). Pinkie mice are quite simply too small for ball pythons.

Some people get way too overzealous about the 'never leave live prey unmonitored' stories. 9 times out of 10, the culprit in these cautionary tales is an adult mouse that is left with the snake for days. The other 1 time, it's a hopper or older rat that's left for days. Adult rodents are dangerous. Some caution is also warranted when dealing with hoppers--hoppers are weaning or weaned, have teeth, and can occasionally be aggressive.

I put hopper mice in with my baby snakes when they won't accept rat fuzzies--I check them several hours later, and pull the uneaten ones before I go to bed.
Crawler rats--those are the rats that have their eyes open, are just starting to get teeth in, but are very wobbly on their legs and aren't weaned yet--are harmless. They are not going to attack your snake. It is perfectly safe to leave one in overnight. Realistically, few brand new hatchlings are going to eat for the first time while you're watching them.

I've never had an adverse incident from leaving live baby rodents in with snakes. Sometimes, if you want them to eat, that's what you need to do.