Could Milo, if he so chose, put one rank in wizard (No capital W)? I know he fears being an unoptimized freakish hybrid, but it seems like it would be a useful class for him to have, given that it suddenly opens him up to all the wizardly magic that he ca

No he couldn’t, because the wanded wizards don’t have classes and levels for him to put points into, in the same way they don’t have hit points. They’re not really the same kind of human. If you looked at Milo under a microscope, you might not even see atoms—he’s made up of the four elements, like all D&D things.