I'e been blending cheeses with my mozzarella for a long time now. Cheddar is a common cheese to add to mozzarella, but cheddar tends to oil off a lot, so if you don't like a greasy pie, stay away from cheddars. Same can be said for some of the Italian cheeses like fontina/fontinella, asiago and sharper provolones. I'm not a fan of sharper cheeses on my pie, the flavor can be too strong. My favorite mix at the moment is mozz with about 15% @ of mild provolone and either fontina or if I can get it, kasseri cheese. I don't really see enough of a difference between most of the well known, widely available mozzarellas to worry about which one I use. Probably my go to standard at the moment is Dragone &/or Sorrento.