The rules are how we engage with the world- the characters wouldn't be aware of the foe's different rules any more than they're aware of saving throws- but they'd know that things weren't right.I'm fine with a foe having new abilities, but having new rules sounds like it pushes on the metagaming barrier. It could be cool if it worked, though.
The players of course would be hearing "make a save vs wand" or an old version of cloudkill
And frankly off the top of my head with the exception of more lethal spells, I struggle to think of how an enemy using, say, 2e rules would have an advantage over 5e characters. Maybe flipped AC or their saving throws being terribly different

Oh, Time Stop and no concentration I suppose