Is there a recommended follower to use for each difficulty level? I completed Normal difficulty just fine with my Wizard class w/ my Templar follower. He seems to be doing alright through Nightmare mode. Should I stick with him or is one suited better for each difficulty?
When choosing a follower, you shouldn't be concerned with what difficulty you're on, but rather how well of a fit they are with your class. For a Wizard, the Templar is a good fit, as he tends to hold agro and allow you to stay back from the action and use ranged attacks. In a similar vein, he's also a good choice for the Demon Hunter.
This choice is obvious for the Wizard and Demon Hunter as you want a melee follower to tank for you. Followers are, after all, just meat shields. For a Monk or Barbarian, the choice is less obvious, but to me the Scoundrel or Enchantress would be better fits than the Templar.
EDIT: Thanks to fbueckert for for pointing out that question and the excellent answer provided by Tater596.