It's interesting that the New International Version uses the word "rebels" instead of thieves in Matthew 27:38 and Mark 15:27. In Luke 23:39-40 it uses the word "criminal".
As for the Tau shaped of the cross there is the minor issue of the sign of condemnation that lies above their heads (Matthew 27:37 NIV). Whatever it is that would have allowed a sign to be put overhead would have given the cross a t shape, not quite a "Tau".