Yes, it is correct. There is a phrase in Russian like "I wanted to buy" = "Я хотел купить", but in case of "I wanted someone to buy" it is not applicable. A complex sentence is needed: "Я хотел, чтобы кто-нибудь купил". I think with "чтобы" and "бы" past tense is used always. I don't know about rules, as a native speaker I never needed to learn them.