I'm buying an FP RED ported and plan to boost about 26 or 27 PSI.....should I order the
18psi or the 19psi Wastegate Actuator?

 

The FP 18 PSI actuator will work fine with the boost levels that you'll be running. I imagine that the FP 25 PSI actuator should work fine as well. although I would only be guessing as I have never personally worked with the 25# unit.