A normal mains switch can be much cheaper and you could have a light to show when its on
If it switches the mains power supply to the fence charger, yes, of course.
If you are using it to switch the energized fence wire, no of course not - that's generally tens of kilovolts, and far exceeds the rating of the switch.
Depends on the voltage output of the electric fence charger.
Given the ranch systems I've run in the past, arc-over in the switch to ground in damp weather is probably a given and so I'd never have considered it. And unless that's a neon light with a resistor that can take the voltage and limit the current properly, it will soon go bye-bye.
If you don't mind getting zapped, you might try it .
There's a reason that electric fence equipment isn't cheaper.
Note: Question has been changed. Originally sounded the OP wanted to use it as the high voltage disconnect.