The fleas shouldn't affect the snake. Same way snake mites shouldn't affect your cat or dogs.
Why would treatig your cat affect the snakes? How are you treating it? Are you just spraying the cat or your whole house? Are you just worried about the smell coming off the cat or are you spraying the tanks? You don't need to treat the reptiles so the treatment shouldn't affect them. If you are just worried about the smell coming off the cat that shouldn't be an issue. Just keep the cat away from the snakes if you think it might bother them. The cat shouldn't be around them anyways. Also the treatment will only be temporary.