20 gal tank - good mix of community fish ?

Thank you on advice to use chemical filter pocket for additional biological filter media, Thor

Frankly speaking I still did not get the idea how you can "train/adapt" new fish to your aquarium water. 
My tap water has 9.4-9.5 pH, there is no way any community fish will tolerate such high pH. At best they can survive up to 8-8.5 pH. 
So regardless of how long I'm going to extend period of adjusting transport container water to the water in my tank the fish will die, it just ca not arrive at the destination - to "hot" for them

Mixing RO water with the tap water is also not helping. Tap water has 9.4 pH, RO (my home RO system) water pH is 6.54.  When I mix 50% tap with 50% RO I get the mix with pH 8.8 (it's not 6.54+0.3=6.84 pH as you might expect) because tap water has KH around 1-2 so some H+ are consumed. 

I believe injecting CO2 is one of the solution. But personally I'm inclining to add small amount of mix of Acid buffer (carbonic acid) and Alcaline buffer (sodium bicarbonate), this way I can target any pH in the range of 6.5 to 9.4