There is a way to keep the typing a mystery! That being, the Goomies don't actually have a comprehension of what "type" means, so if they're asked what type they are, they will respond with their current mood, I.E. —

"What type are you?"
"HAPPY. C8"

Or other such misunderstandings.

(They will also have a vaguely French accent, not that the Pokemon will actually understand what a French accent sounds like because lulz, Pokemon.)