If you have some "ordinary" diodes laying around, you could put them in so they clip the piezo signal at ~600mV by routing the excess to ground when they conduct in the forward direction. Of course you'll have to account for it in the arduino when you read the pin value with the ADC. This should help greatly with crosstalk on the channels. Like I said, just try it on one piezo. This is the one you hit to test it of course. Your 1M resistors are in series (piezo->resistor->ADC pin), not parallel right?