I don't think I've seen a link to this site in this thread yet:

Thalys | The high-speed train between Paris, Brussels & Amsterdam/Cologne

I found this guy's site extremely helpful in planning my travel. Perhaps it will clear up some of your questions. Everything from where/how to arrange tickets to where the best seats are on the trains.

I don't think the Thalys trains operate from Mainz/Frankfurt. In Germany, they originate out of Cologne. You could either make your way there from Mainz and then on to Paris or just take the normal DB trains and forget the Thalys.