The problem with the climate argument is that you can recruit Southern Gallic swordsmen in Camulsoladae (spell check) and Ictis, and the climate there is much like Normandy, Belgium and Brittany. (IIRC, the reason for them being there was the migration of southern gallic tribes and belgaic tribes to the region and being known by the same name and cultural oddities as some of the mainland tribes).

Btw, does anyone have any answers to my later question?