Ah yes. I actually have that one (and the Persian that goes with it) but I couldn't be sure if it was Delta or just Meowth were part dark-type. (They can use dark-type moves in the later games, such as bite and pursuit.)

It is entirely possible for Pokemon to mutate into various Delta types, particularly as you are the one making the rules in this particular scenario.

Gaius would have a Shuckle. It could help him create potions.