How about some java moss as a carpet? It seems to be a good palnt for a piranha tank.
Also, I know I'm being picky, but move back the swords a few inches. It will help keep the piranha in the front of the tank so you can see them better. It might be just because they are small, but in the zoomed out picture, I couldn't see them at all. Piranha usually sit out in open water in a group of 6 or so. At least, thats always how I recall seeing them in dispalys at the lfs.
Yes piranha are very shy. IMO, their aggresion is really from their fear of everything. The ones I had would hide if you even went close to the tank. Tapping on the glass makes them go nuts. And if you corner them, they will attack you so always watch where they are when you clean your tank. They should stay away unless they feel cornered.