Can this rail be added on to the 75" plunge saw guide rail to make a reliable 107" rail guide?

Yes. Festool guide rails can be combined to extend their length. We would recommend that you use a straight edge to double check alignment. You would also need two (2) of the rail connectors, linked below.http://www.festoolproducts.com/festool-482107-guide-rail-connector-fits-all-rails-2-required.html