Hi Daniel,
For the gear-driven FLEX tools like the, BEAST, Supa BEAST and CBEAST I preach, teach and practice Speed Setting 6 - get in and get out.
For any free spinning random orbital polishers, like the Porter Cable 7424XP, you're also going to be on the 6 speed setting. You want to maintain sanding disc rotation for the fastest and most efficient sanding action and with a PC that's going to be the 6 setting.
For any other free spinning random orbital polisher like RUPES, Griot's, HF, etc. you want to see sanding disc rotation, not sanding disc stalling - so experiment and bring the speed up until you see good sanding disc rotation as you move the tool over the surface.
For wet sanding by hand or machine - yes, add a few drops of car wash soap. The soap helps to lubricate the surface as you sand better than just water. The soap also helps prevent the face of the sanding disc from loading up.
For everyone reading this into the future - nothing wrong with going it alone - but when you see my next boat detailing class coming up - clear your schedule and get signed up and let me completely flatten out the learning curve.
-Mike Phillips