@heavanet- If you decide to make your cheese soup with hoop cheese, I don't think you will be disappointed. Hoop cheese is easy to cook with, and melts into a nice, creamy texture. You can add milk or any other ingredients that you prefer to get the flavor and consistency that you like.
Basic cheddar cheese can be quite lumpy when you heat it, so I think that you will find that hoop cheese is even easier to use for making soup. It is also not too bold in flavor like some cheddar cheeses can be, so you won't have to worry about the flavor once your cheese soup is done cooking. It should be mild and creamy, just like you want.