Citrus juicers, as their name suggests, juice Citrus Fruits. So Lemons, Limes Oranges and Grapefruits, and all similar fruits are to be used in this device. A centrifugal juicer however can juice most non-citrus fruits, as long as the fruit has juice...
If your smoothie is too thick you need to add more liquid, all smoothies have to have a liquid base so it is best to use the markings on the goblet for best results. If your smoothie is too thin then add some frozen ingredients or maybe half a banana.