Beast Boy himself would be op in the Pokemon universe. Both the Trainer based one and the Pokemon Mystery Dungeon one. He does not just transform into animals, but also replicates their abilities as well. Since Pokemon are sapient yet super powered animals basically this includes them as well. It's not just Moves or Abilities, but passive noncombat abilities too. This of course scales with Mythicals, Legendaries, and Ultra Beasts as well. Not to mention he can directly train with any Pokemon directly by transforming into them to give specific training maneuvers. Plus it would help bond with them more.
Personally I would have had Zoroark (Dark), Hisuan Zoroark (Normal/Ghost) since they both transform through illusions, or Flygon (Flying/Ground) on his team as options. Also if not Rillaboom there is also Sceptile or Meowscarada.
