CO2 will reduce your pH by the same amount no matter what your KH is. To get CO2 to recommended amounts, your pH will have to drop by 1.0 to 1.4. A drop of 1.0 will give you around 15ppm, a drop of 1.4 will give you around 35ppm, this is assuming that when you test your pH prior to injecting co2, you are near atmospheric equilibrium around 2.0ppm. Increasing your KH will increase your pH, so you can start higher, and end up higher in the end; but this type of pH change does not affect the fish much, if at all. It's TDS that the fish care about - KH and GH being part of it. Increasing KH for the fish, if they are fish that like harder water, is fine. But increasing KH so that you can inject co2 and end up with a higher pH on your test kit after you inject it, is just messing with numbers and perhaps hurting the fish if they are fish that like soft water to start with.
A cheap way to increase GH (which plants do need) is to add epsom salts for magnesium and calcium chloride (pool hardness increaser, available at pool and spa shops) for calcium. Then your bases are covered as far as GH goes.