Wow. Wow, wow, wow wow. I am so sorry, OP. This is ridiculous, and is definitely a black mark on the adoption agency.
From the sounds of things, there's nothing in the contract that specifies what sort of pets you may or may not own in order to be eligible to adopt a cat. Nada. Not a thing. I would point this out until you're blue in the face. Contracts also tend to be legally binding, so the fact that they allowed you to take the cat home and approved the paperwork before trying to revoke your cat would not look good for them in a court of law. I think at the very least threatening legal action would be a good idea, and it might even be good to contact a few lawyers in the area just to back you up if need be.
Do you have locks on your ball pythons enclosures? If they agree to a wellfare check, having locks on the enclosures themselves could only help them feel more reassured that the cat is indeed safe.