It can view other skins, but you have to save them to a folder first. Example, inside AH hangar right click on the plane after you pick a skin and save the skin. Then it makes a folder with BMPs in it (which the skin viewer can see).
Or open the viewer, find any plane, hit "Save" and then rename it to xxxxxx1 (where xxxxx is the default name). If you have more than one directory for a certain plane that have skins in them, you will get more options in the second combo box (right below the one where you pick the plane you want) in the skin viewer.