Possums, in my opinion, are lazier predators. In a choice between the cat food left out on someone's porch or a chicken, they will take the cat food. So I don't consider them as much of a threat as say, raccoons, who will just happily go straight to your chickens and eat what they want and kill the rest for the fun of it.
Oh, and raccoons WILL be a threat to you as well, while you're at the hardware store grabbing the hardwire cloth, grab some locks for your coop and run doors as well. Coons can work latches.