Cool, thanks so much for that excellent information!

I've got a temporary jumper from the 5v output pin of the Pololu to the 5v pin of the USB connector. It works and seems to do what is expected, though I haven't tried it when things are cold yet, this is all on the test bench. With it powered by the Pololu like that, can I still safely hook up a USB cable to the computer simultaneously? Sometimes I have to debug while things are powered.

I think that long-term, if the Arduino Mega board is going to be my 3.3v source, then I need a 7v power supply instead of a 5v power supply. So I could swap out the Pololu from a 5v one to a 7v one, I'll bet they'll have one with the same pinouts, I could look.

Would another option be to get a small linear regulator that takes 5v in from the existing pololu and puts out 3.3v, much similar to the one on the Betz board that takes 5v and turns it into 2.5v?