Don't be concerned about the pH. CO2 can't drop the pH enough to harm the fish - they would be dead from suffocation long before the pH drop could concern them.
Plants need calcium and magnesium, so GH shouldn't be too low, and you need to make sure your GH consists of both calcium and magnesium, not just calcium. With your low GH you could just use Seachem Equilibrium to add enough magnesium, or add calcium sulfate or chloride and magnesium sulfate (epsom salts) to get more GH.
We used to believe that KH had to be above some minimum number, but now many people, including me, don't bother with KH at all. Many plants grow much better with low KH, and fish have no problem with low KH.
Neither KH nor GH should change with addition of CO2, but both can drop from the plants consuming the calcium and carbonates in the water, if you don't do regular water changes.