Some services actually start in Glasgow, and so Edinburgh may not be the first stop. If you have a reservation, you are entitled to ask anyone sitting in your seat to move. However, depending on when you travel, you will probably find that there are plenty of other seats available too i.e. non-reserved seats and reserved seats that are not being used. If the reservation ticket on the seat states that it is reserved from a point before you got on, but the seat is empty, it is safe to assume that the seat is free. But often the ticket will show more than one reservation for a single seat (i.e. Glasgow - York York - London etc.) so you may want to check that too!