would splicing the wire from the OEM water temp sensor be bad? i wanted to splice the oil psi, and water temp but theres 2 wires, green and red and i dont know which one i should use

Just solder the connection, and tape/heatshrink it up good! DO NOT USE T-TAPS OR SCOTCH LOCKS! they ruin the wire, and leave the possibility for corrosion since the metal is not sealed from the elements.

geez i dont know if thats a good idea, the voltage going from the water temp sensor to the PCM is pretty important.

You will not lose any voltage, unless you are adding a high current draw. If you are just adding a guage, I can't see it being a bad thing, as long as you take the precaution to not allow the circuit to EVER see water.