No you can't just use the stock sensor. The guage and sender are calibrated to work together.
Did the guage come with any adapters? If not you should be able to buy one from Enzed, Pirtek, hydraquip etc or even a brass wares store.
Temp sensor is normally on the drivers side of the thermostat housing. The other one is hooked into the carby/emmissions control stuff somehow.
The headunit power wiring should be on the accessories position of the key. You should have the guage on "on" power.
If you wired the guage up with the headunit then when you're sitting still listening to the stereo (with engine turned off and key on Acc) your temp guage would also be on.
You should be able to pull some "on" from the back of the fuse box easily.