So you need a thermostat to let the engine run at a reasonable and stable internal temperature. Hopefully you didn't just remove the thermostat. The dash panel, that is now gone, displayed the internal engine temp and not the temp that turns on the fan. They are separate and operate independently.
The fan & switch are a simple relationship. There should be no real reason to add a switch unless it is running too hot and you want to run the fan more frequently. If i recall correctly, the thermocouple (switch) that turns on the fan is, or was, in the left radiator. You should still use it, but not to control the "engine operating temp". That has to be controlled by the thermostat.