I think I would just stick with the 6 danios. If the red tail you are referring to is a red tailed shark, you need a much bigger tank for that fish. When you buy fish, consider them for your tank at their adult size. Cory cats are happiest when they are in groups of at least 3, and 1 would be very lonely. I think 6 danios are probably gonna do it for your tank. They are very active and fun to watch fish. Don't add all 6 until your tank is cycled. Please read the articles here at FishLore about cycling your tank.
