This idea has nothing to do with shields, and yeah I don't see how this will cause more beam spammers. I think it will acually reduce the amount of beam spammers because people will be more careful using their ki's.
Yeah why not put him in? It's pretty balanced. If you don't like Tien then don't choose him. Makes sense doesn't it. Also with this idea it can make Tien a lot different from others. Everyone in the game should be special in some way.