Sorry this is a stupid question but I really need to know because I'm challenging Erick's gym and apparently Politoed was banned, so now I'm at Sand xD And I looked up PO Monotype rules, and the post there was from 2011. So I'm not sure if it was updated or not.
Just tested and yes you can however I would advise otherwise. It is walled by Mono Steel (most common type of team).
