Awesome idea. I'm now thinking about a hack of your hack in which the monsters aren't heroes (except to themselves), and the main enemy is the human world. King Kong, or Cloverfield, for instance. Players vs the MC's fronts (which are mostly human combatants) and someone's probably gonna get killed, or driven back to their lair to await the sequel. (ETA: Or PvP - monsters vs humans - with the MC functioning as Referee.)
Cloverfield is a Lizard (nervous, territorial, amphibious), and the Parasites are a Swarm (speedy, spread out, toxic). But they are not working together, their goals are entirely self-centered (at the beginning of the movie the big monster shakes off a bunch of parasites which had been burrowing into its skin, thus freeing them to ravage the city of New York, and after that scene they pay no attention to each other at all).
FYI: The first RPG I ever bought and GMd was "Monsters! Monsters!". :-)