What is the correct usage of this site? Should I be using this for all my CS50 questions, or are some not allowed? What about overlap with Stack Overflow?
I'm gonna quote What topics can I ask about here?
If your question is not specifically on-topic for CS50 Stack Exchange, it may be on topic for another Stack Exchange site.
Of course there's an overlap with Stack Overflow. I like thinking of this site as a more beginner-friendly Stack-Overflow-like website. This site probably has the first priority for questions that are mainly about the course's materials and/or psets.
It's probably easier/faster to get answers for such questions here than on Stack Overflow since technically most (if not all) the members of this website already took/are taking CS50.
Keep also in mind the "Academic Honesty", it is defined here.