Jessie, regarding your first question, these tombs actually were generally reserved for kings and other people of great importance - in fact, I have never heard of another tomb that was built for any similar kind of reason, which makes the Taj stand apart.
Shah Jahan actually intended to build another tomb in black on the opposite side of the river for himself, but it could be said that he forfeit his own tomb for his wife in that he later decided to be buried with her in the Taj.