I want to make an app to teach Chinese and prepare people for HSK, but I don't know if there are any rules or regulations that I should adhere to.
Probably you should check out the official website and see if they impose any regulation on preparatory materials, I guess they don't.
I assume, to be fair, you won't be able to advertise your app as 'approved', 'official' or 'everything you need to know'. After all, the vocab lists they provide are not sufficient for the test, at least in some levels. Reading texts may contain vocab items that are not listed. And if your app is only a vocabulary trainer, it's quite far from comprehensive, since examinees still need grammar, expressions, writing skills, fast(!) reading skills, hearing comprehension, etc.