It sounds like they kept the rule vague on purpose. I would clarify it before you buy your snake. As for neighbors knowing you are keeping snakes I feel that it is better for them not to know. All it takes is one person to be afraid of snakes making a big fuss and you have a serious problem.
I would put Ball Pythons in the medium snake category, however some of them can be on the big snake list for me. Such as any over 1700 grams. I wish you luck, and you may want to start looking for a new place to live so that you can keep your snake without a hassle.