The tour company we are considering includes a rental car. Do we need any special licensing or whatnots to drive around fro Frankfurt to St. Goar to Ansbach, etc?
No. Any valid US driver's license will do. Also driving in Germany isn't much different from driving in the US. Most of the cheaper rental cars are manual (stick) only as this is more common in Germany anyway. Diesel engines are more popular as the fuel is cheaper, acceleration and milage is better.