Repeating my end point from Part 2, I'll assert that in general one cannot have a "monster rally" if one has just one type of monster versus another type of monster. Examples of these would the meeting of "werewolf star" Waldemar Daninsky with assorted vampires in FRANKENSTEIN'S BLOODY TERROR, or the encounter of the Big G with one-shot nasty insect Megaguirus in GODZILLA AGAINST MEGAGUIRUS.
However, it's a different thing altogether if one has more than one monster in an ensemble constellation. In GODZILLA'S REVENGE, both Godzilla and his son Minya form an ensemble of two. Though they have a common biology, they are not as identical to one another as, say, all the vampires in BLOODY TERROR are, and so they make up an ensemble of two fairly distinct characters. The feeling of the "monster rally" is further enhanced by the fact that the duo take on at least three other creatures on Monster Island, but only a single opponent is needed if one is dealing with a multiplicity of centric monsters. The latter case appears in the Toho film immediately preceding REVENGE: DESTROY ALL MONSTERS, wherein most of Toho's monsters take on King Ghidora.
However, a one-on-one "monster rally" is possible if one is dealing with a situation where the two creatures have sustained their own "centric" stories. KING KONG VS. GODZILLA was one of the few Toho films that qualifies for this "honor," while others include FREDDY VS. JASON and ALIENS VS. PREDATORS.
It's also possible to see the narrative structure of the monster rally when there is one "starring monster" allied against several non-centric types. FRANKENSTEIN'S BLOODY TERROR does not qualify, but 1969's ASSIGNMENT TERROR, which pits the wolfman against both a mummy and a doppelganger for the Frankenstein Monster.
It's also possible to see "teams" of monsters opposed to ordinary humans, as in 1943's vampire-and-wolfman team for THE RETURN OF THE VAMPIRE.
Or conversely, one may reverse this structure. In ABBOTT AND COSTELLO MEET FRANKENSTEIN, where the demihero-characters played by the comedians are the centric stars of the show, and the three monsters are their non-centric opponents.
A similar dynamic holds for heroes as well as demiheroes. Marvel superheroes are the stars of the one-shot comic MARVEL MONSTERS: MONSTERS ON THE PROWL, but it's still a "monster rally" because they're pitted against a mess o'monsters who originally had separate story-arcs in old monster-comics.
There are also a number of situations where the story concerns a team of "good monsters' versus a team of not-so-good ones, as seen in the game-turned-cartoon DARKSTALKERS.
However, if you've got one team of monsters for good, you don't need a team for evil to have a monster rally, as witness that salute to 1950s fiends, MONSTERS VS. ALIENS.
All of these examples involve some strong life-or-death conflict. However, there are also various stories which follow the "domestic comedy" pattern. Thus, in one cartoon special, the demiheroic Flintstones meet a monstrous family in THE FLINTSTONES' NEW NEIGHBORS.
And this, of course, was a direct swipe of one of the earliest "domestic monster rallies" in popular fiction, THE ADDAMS FAMILY.
So, in all, I count ten distinct storytelling variations which manage to cross over more than one distinct monster-types-- which is probably the most attention that anyone has ever devoted to this perhaps deservedly arcane subject.
ADDENDUM: I should add that there's one exception to my rule about "fairly distinct characters." This is when the monsters all have the same origin, but they are BASED on originals who were distinct. Thus in the movie SCOOBY DOO 2, a scientific process creates monsters who look like some of the costumed villains who appeared in earlier SCOOBY DOO TV episodes. This also applies to dreams, in which a dreamer simply dreams about a bunch of monsters that have their own existence in the "real life" of the ongoing narrative, or when human agents impersonate a bunch of monsters that were supposedly real at some time-- which itself sounds like a SCOOBY DOO episode.