Caputo 00 is a soft wheat as are most European flours, to my knowledge. The blend of flours, Caputo + semolina, is something I got off of the Internet.I use the 1 egg/100g flour. It's foolproof. Never tried Ruhlman's because the other is foolproof.
Not sure about mixing low gluten Caputo with high gluten semolina, though. Typically I use AP and egg for fresh pasta, and semolina and water for extruded. If I thought Caputo would make a significant diference I'd totally sub that for the AP. I have never heard of anyone doing it the hybrid way you mention... but that may just be something I just never heard..........................
FWIW I substituted White Lily AP (soft southern wheat) for KA AP in my last bread dough and the resulting texture was much much softer.
500g KA Bread Flour (hard wheat)
300g White Lily AP (soft wheat)
100g whole wheat or rye flours (any combination thereof)
590 - 610g water
3 TBS olive oil or lard