Females have no venom, but males are known for being a bit agressive, especially during the breeding season, and then they have a pretty nasty venom- so at least at that time, I’d expect a male to win, if a fight started. The cat could just stay out of the way though- platypuses can hardly climb so well, and aren’t very fast on the ground.
The rest of the time, probably the cat would win, if they actually fought. The platypus would probably just try find somewhere to hide, the cat might well be curious, and start batting at it or trying to kill it- depends on the cat.
I went through a jungle survival school in the military. Guys were always catching critters and bringing them back to the barracks. There was a wire cage outside the barracks for them to throw their new pets for some Roman Coliseum type fun. I saw monkeys, boa constrictors, buzzard chicks and sloths in there. They all pretty much retreated to a neutral corner of the cage and shivered in terror at having been captured by human beings.
There was also an alligator pit where the critters would eventually be thrown after they had failed to fight each other to the death. The alligators always provided the thrill that everyone craved.
I don’t know, but I do know for a fact that cats and opossums get along just fine. I’ve seen them eat out of the same bowls together on several occasions(different animals). Strange, but true.