CO2 can't lower the pH much below 5.5, if that low. At normal aquarium water temperatures, that is about the bottom limit for pH due to CO2. (I think this is because no more carbonic acid is formed once the pH is that low.)
If your KH is above 1 degree of KH you have as much buffering as you can get from KH. Increasing the KH further just raises the pH.
It is safe to raise the KH with baking soda, but not necessary to raise it above the 1-2 range. And, once you start adding baking soda to adjust the KH, and GH booster to adjust the GH, you are stuck with having to match those adjusted values with each water addition to the tank. Of course small variations aren't a problem, but adding half a tank of 1 dKH water to a tank of 6 dKH water is not a small variation to the fish. It is an abrupt drop in half of the KH. This still might not prove to be a problem, but how will you know for sure?