Aha, a monoteam, sounds like an awesome idea! The available Steel Pokémon are Empoleon, Scizzor, Magneton. Sleetix, Bronzong, Bastiodon, Probopass. If you can't get a Shieldon fossil in the undergroud you still have 6 other steel types. And all of them has a secondary type, so you also have some variety!