20 gal tank - good mix of community fish ?

I also have a question on pH control.

All cardinal/Neon tetras, Corydoras and Gourami require pH to be 7.0-7.5 max.
My tap water is really soft, it's 1-2 KH, 1-2 GH and 9.4 pH. So in order to keep the fish I need to target pH of 7.
My plan is to use combination of Seachem Alcaline buffer and Acid buffer, this buffers are plant compatible. First get amount of Alcaline buffer to target KH of 3-4, then add/mix Acid buffer to target water pH of 7.

Thor, could you comment on the plan and suggest an alternative.