I'm not looking at it at the moment, but I THINK you have to add a "behavior", too. It's probably "SHIELDED", but look at the behavior lists of some units with shields, to see if I'm right.
No, you won't have to create a new armor type, though. Shields have nothing to do with armor types. (They have their own.) You will have to add a line for that, though. I think it's something like "<Shield_Type> Tank_Shield </Shield_Type>". Again, look at an example first, since I'm typing this without checking the XML files, myself.