Hi Dave...welcome!
The only popcorn maker I've ever roasted on is a Whirley Pop, back in the day, so I can't speak from experience. But, I equipped that with a probe, and the basics are the same, whether its a popcorn maker or a large shop roaster. You want to get the probe in the center of the bean mass as much as possible. For a fluid style of roasting (assuming you're using the air popper version) that's more of a judgment call, and you'll end up measuring air temps as well, but with proper placement it will allow you accomplish the same results - repeatability.
The ThermaQ Blue accepts ANY style of K type thermocouple, and thermocouples CAN be bent AS LONG as you bend on a radius, i.e. no sharp corners. Think "U" shaped, not "L" shaped. Also stay away from the tip (where the junction is) at least 1/2". That being said, I'd take some measurements of the machine...maybe use pipe cleaners if you plan on bending it to help you get it right, and choose an 1/8" K type probe that hits your measurement plus some to go through the casing. You can get a wide array of sizes on Amazon...ranging from an inch or two through longer lengths.
Good luck!!