Agatha and the boys as the Librarians in training,

Zeetha as the Guardian.

But then i was thinking who would be the librarian, but then it hit me.

It would be Higgs, Higgs would be the librarian. The unflappable incredibly experienced, but considers himself unqualified Librarian.

after that everything else just fell into place, The castle is the Library, albeit its more neutral than hysterically evil, and unable to work at full capacity, (perhaps because a Hetrodyne isn't in residence.)

Barry was the last Librarian before Higgs, but left to go on his own, unknown mission.

And before him Bill was librarian, with Klaus as his guardian. Then Klaus got kidnapped and everything went to hell.

The Receptionist is Mamma

The Butler (jeeves) is Carson

and for lack of a better fit, The mirror ghost is General Khrizhan, mostly since mamma and him have a history.