The more I read the rules on this, the more it seems like you are just going to have to learn it by repetition.
It seems like Dutch's elevator pitch to English speakers is: "Do you enjoy the arbitrary spelling rules of English, like I before E? What if your language HAD MORE?"
Here "weinig" is not an adjective. It is a quantifier, that is why. If it were preceeded by a "de" or a "het" then we would be talking about a specific group of things, and, therefore, it would accomplish the grammatical function of an adjective, like in "De weinige interessante boeken zijn duur".