I never heard of either milk and Seven Up or Milk and Pepsi. I really led a sheltered life!
I did, however, happen upon a mixture on my own many years ago which I still enjoy today and that is Diet Coke and coffee. It happened for me by accident. I had some black coffee left in my cup and came back to my office with a can of Diet Coke and added it to my cup without realizing there was coffee still in it. I was pleasantly surprised by the combination, which tastes similar to cream soda, and I still mix the two today.