I think the reason Noel is allowed to keep his Clefable was because before it was a fairy type, it was a normal type. Plus, it is a wildcard which makes it an added challenge. Also, for Hippowdon, Florinia isn't a grass gym leader, but her team is based around sand while with some added grass types. Hippowdon would make sense given her sand field. However, I don't really see anything wrong with needing a 'wildcard' mon if that's how you feel. However, it isn't truly a monotype run if you add a wildcard to the mix, in my opinion anyways.
It's still your game though, and you play how you'd like. There's really no right or wrong way.