The 1970s Gold Key series THE OCCULT FILES OF DOCTOR SPEKTOR proves pretty rich in mashup scenarios, though not all of them qualify according to the categories I laid out here.
The main character is an expert on the supernatural who runs around investigating spooky stuff, even though he's not really a man of action and has no supernatural powers himself. After meeting versions of Mr. Hyde, the Frankenstein Monster, and the Mummy, and an "original" vampire antagonist named Baron Tibor, Spektor has an encounter with a group of vampires. Technically this can't be a mashup since all the vampires share similar origins, but it's worth noting simply because it teams up the best-known literary vampire, Count Dracula, with three others from prose lit: Sir Francis Varney (VARNEY THE VAMPIRE, Mircalla Karnstein (CARMILLA), and Lord Ruthven (THE VAMPIRE). Baron Tibor gets involved, but on Spektor's side.
The first true "monster mashup" in the series is issue 9, but it's an illusory one in which a witch causes Spektor to dream that several of his foes are attacking him (the "vampire" listed on the cover is the series' original creation Baron Tibor).
In issue #11 Spektor is afflicted with a werewolf curse, and in #12 he has a fight with the Frankenstein Monster that's clearly an homage to FRANKENSTEIN MEETS THE WOLF MAN. I can't consider this a strict "mashup" though, since Spektor is in the position of the main character and he's only temporarily a werewolf.