Spirit is the first FBL unit I have used that ask for the Throttle Frequency. How do I determine what Throttle Frequency to use with each ESC I own? HobbyWing, Jeti Mezon, Castle EDGE, etc.

You should check specification in the manufacturer. Each should specify it.
Some are recommending 66Hz as max, some can handle 333Hz without any problem.

200Hz is working with the most ESCs. Frequency has great impact on the governor performance so it is good to set it as high as possible.

For HobbyWing 200Hz is good, for Jeti 120Hz, for YGE 200Hz, some Castle ESCs can accept only 60Hz.

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Spirit System developer

What happens if the Throttle Frequency is set too high? Will the ESC catch on fire?

No, it shouldn't. I can't guarantee it, but rather it will behave in a strange way. For example:
- it will not see throttle signal at all
- response for the throttle change will be inadequate
- internal spoolup of the ESC (if used) will not work properly.

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Spirit System developer