Actually, almost everyone has one foot slightly longer than the other. The difference is usually less than half a shoe size so it does not need to be addressed in most cases. The longer foot is usually the opposite of your predominate hand, so if you are right handed your left foot is longer. In some cases one foot can be thicker than the other, usually the same side as the predominate hand, but not always.
Most people are surprised to find out how different their feet really are the first time they get measured for bespoke shoes.
Complicating this whole foot length issue is the fact that shoes are designed to fit from the back of the heel to the ball of the foot, not to the end of the toe. You want your heel to sit in the heel cup as best you can, so I would suggest a tongue pad as opposed to a heel pad or a padded insole.