It depends on what kind of chaos and mayhem Varian will actually unleash.
If Varian for example behaves like a bomb maniac and throws around dangerous poison gas flasks, or activates black hole machines to destroy the capital city of Corona, then I seriously doubt that Monty would ever team up with him. Monty's the most beloved and friendliest guy ever, after all.
But if Varian's villainy was just restricted to writing angry letters to Rapunzel and starting s smearing campaign, then yes, he'd probably help him.
Perhaps even join him in a hearty >BOOO
against the princess.