I'm in the middle of some of this myself, so here is my $0.02.
I don't think hooking up your fan temp sensor to your gauge sensor will work because I think your gauge sensor would be sending varying voltage to your gauge depending on temp so the gauge's needle would move.
Your fan temp sensor is more of a switch (on / off).
You may want to try buying an adjustable thermostat ($17 or so at AutoZone) and having that control your fan. You can either run the fan power straight to the thermostat and use that as the power switch (the wiring scheme that came with in the package), or wire in a relay and use the adjustable thermostat as a switch for the relay (what I did since I have a Haywire wiring system that came with a fan relay). An adjustable thermostat will let you control exactly when your fan comes on.
You can put the temp sensor in your radiator's cooling fins or on the motor side of the radiator.