I wouldn't put a cat or dog with my ball pythons. It's just too risky. What happens if the dog/cat is in a playful mood and tries to nip/bat at the BP? Of course the BP is going to react, then soon you'll have a sticky situation where you need to decide which course of action is going to hurt whatever pet the least.
You're going to have this snake for ~20 years. You will gain it's trust, just give him time to settle in and get used to some things.