Amano and Cherry Shrimp will do fine in waters of higher GH/KH, however the swinging of parameters you have is likely causing unwanted stress. What's happening is the ADA Amazonia is trying to keep the pH low and the Seiryu Stone is working to raise the pH. Because of this, Ca, Mg, (GH) and carbonate (KH) are being released from the stone. I had this same issue in a Spec III, ended up taking down the tank.
Here's the options I see:
1) Remove the stone (not too much work, could find a different type of rock, one that wont effect pH)
2) Remove the substrate (change to an inert substrate, could be a pain, not sure how big your tank is)
3) Deal with it as is, hope everyone doesn't become too stressed.
4) Try and find a balance in parameters. Go longer between water changes and add minerals into the RODI water before adding to your tank to help buffer against GH/KH swings. @Zoidburg
is correct, this will exhaust the buffer capacity of your substrate. At which point the pH will stop dropping and level off (my guess is around 7.5, idk tho...).