If I were you I would do a big water change, forget the buffers, add CO2 to get the amount you want, and whatever the KH, pH, and GH end up at is fine. The fish you have are not demanding fish that need specific hardness or pH.
I'm not at all sure the pH, KH, CO2 chart is accurate when using a Carbo Plus unit. That unit, as I understand it breaks down the KH to get CO2 from the carbonates. If so, it will drop the KH as it drops the pH, so both would have to be measured at the same time and with the Carbo Plus unit operating.
The argument against the Carbo Plus method, as I see it, is that you pay almost as much initially for the unit as a pressurized CO2 system costs, but still have to buy new carbon blocks periodically, which are expensive. But, the CO2 system uses very cheap bottled CO2, so the first expense is really the only expense. Both systems are virtually automatic once they are adjusted. However, the idea behind the Carbo Plus unit is intriguing, and I suspect it would work adequately for a small aquarium. A big advantage of the pressurized system is that so many of us use it you have access to lots of good advice about it, and that isn't true of the Carbo PLus.