1. You can. In fact people do. Longer focal lengths have the advantage that you can (for the same magnification ratio) remain further away from the object you are taking the photo of than with a shorter focal length. This is useful as you will have more light on the object. (Sufficient light is one of the problems in macro photography. Why this is so needs a long explanation.) 
2. Macro is not tied to the focal length.
A macro lens is a lens that is calculated to give best performance for short film/sensor-object distances. In fact you can take any lens and insert macro rings to get nearer your object. But a lens not calculated for short distances will deliver a subpar image.