When do I need to use subcategories?

Sorry if these are dumb questions, but I haven’t found answers in the forums:
1) If I want a page to display Indian Restaurants, do all of my Indian restaurant listings have to have Restaurants as a Category, and Indian as a subcategory (or vice-versa), or can I just assign them to both categories?
2) If I want to create similar pages for Indian markets, and Indian community organizations, (or Italian, etc.,) do I have to recreate the same subcategories under each category – e.g. – Indian as a subcategory under Restaurants, and also under Markets and also under community organizations? Or can I just assign them to both categories?
3) If I want to have a page that called Indian Twin Cities that displays Indian restaurants, Indian markets and Indian community organizations all on the same page, (or Swedish Twin Cities, etc…)how do I do this? Can I just create a category called Indian Page that I assign to all of my Indian listings regardless of what other category (restaurant, market, etc.) they are in?

Thanks,
Jeremy