In my (extensive) experience, cats are indifferent to possums. A strange cat or one who has been “voted off the island” shows up? There’s likely to be a fight happening. A possum of any size shows up? It gets a glance, then gets ignored.
I have inside/outside cats–and possums always manage to come in with them, more times than I care to count. When I catch one (by the tail–they are slow-moving and not very bright) I would occasionally try to annoy them into “playing possum”, but they never did it within the limits of how much effort I was willing to put into being a jerk to them.
So the one exception to my “cats ignore possums” experience came once when a cat had a nest of young kittens underneath a piece of furniture in my living room. An adolescent possum wandered into the room and happened to walk past the spot. The possum was minding his own business, completely unaware of the kittens. But the mother cat wasn’t willing to give the benefit of the doubt. She moved so fast that I literally could not see her coming. One moment, I was watching a possum walk across my living room–the next moment, there was a slightly larger cat with all 4 legs wrapped around the possum. And the possum was instantly out cold completely possumatonic. I untangled the cat from the possum, took the possum outside, and waited to see what happened. After a minute or two, the possum recovered itself and ambled off, probably wondering what the hell just happened.