We have a 11:20am Friday flight out of Frankfurt to US (no luggage to check) Is staying night before in St Goar ok? Don't mind a 6am train. Can we buy the tickets at the station the night before?
Yes. You can buy 19.50 EUR pp regional tickets to the airport. Regional train to Mainz, then S-bahn to the airport. The 6:19 will get you there by 7:59, and you have back-ups at 6:45, 7:19, 7:56. They all take between 1 1/2 to 1 3/4 hours to the airport.
"Can we buy the tickets at the station the night before?"
Yes, but be certain you are buying for travel on Friday's date and not Thursday.
You will have a change of trains in Mainz (or possibly another station depending on departure time.) Be careful to switch to a regional train there - "S" or "RE" (not the ICE or other high-speed train) or your €19.50 tickets won't be valid.