Prestone coolant is what I have been using without any issues. The only time i used RTV silicone was when the gasket was paper material. The metal gasket doesn't require any sealant.The rtv applied was not on the entire gasket, just in a few spots to hold in place. That might be the issue. Will have to figure out a way to keep in place for the next one. Did not use a torque wrench. Ive honestly never used a torque wrench on things like water pumps. I did use a star-ish pattern and gradually tightened in a star pattern. I don't think it was an uneven torque issue causing the leak.
Try to get OEM water pump (korean made) some Gates wp are hit or miss (chinese made). Torque wrench is a must i think.