Quote Originally Posted by ark
Emiya and Hercules could have easily had been a saber, just how Gilgamesh could have probably rolled any other knight class as well.
Well yeah, that part was integral to what I was saying (how you can choose). There's no point in choosing if your hero can only be one particular class. As you and others have pointed out, there's really nothing backing that up though besides some coincidences that can more readily be explained by hero artefacts rather than incantation tweaks.