OK, that is equivalent to about 8 dGH, which is considered soft to very slightly medium hard. This will be fine for the fish mentioned. Now to the effect on pH.
As fish1983 mentioned, the pH is a bit high. However, two things. First, commercially raised fish are usually adapted somewhat to higher pH which is reflected in the range for pH in the profiles. Sometimes the fish may not be as colourful even though it will basically manage at the higher end of the range, and that is noted in both Glowlight and Lemon Tetra profiles. But here we come to the hardness; it is not high so all else being equal, the pH in the aquarium will tend to lower a bit over time. This is due to the natural biological processes that acidify the water. The degree of KH (carbonate hardness) "buffers" this somewhat, but with a low GH I would expect (though this can vary) the KH to be comparable. So. as long as there are no calcareous substances (limestone, dolomite, marble, lavarock, coral) in the tank, this should occur.
Adding real wood also assists this process, and the more wood the better with Amazonian fish. Sunken wood is abundant in many of the streams, and certainly in the flooded forest ponds and pools. This will aid in lowering the pH by natural means, which is the only way this should ever be done.
The degree to which the pH will lower depends upon the KH of the water and the amount of wood. If you wanted to, you could use prepared water instead of tap water for water changes. With a 20g tank this would not be as cumbersome as it would for larger tanks. By prepared I mean using rainwater, distilled water, RO (reverse osmosis) water. I don't consider this necessary in your case, but it is another method. Keeping the water changes small, say 30% of the tank weekly, will also help to maintain a lower pH over time. And with plenty of plants and not overstocked with fish, water changes can be minimal. Some planted tank authorities recommend no water changes for months, and this can work if well planted and with a moderate fish load.
Hope this helps to explain things. As other questions arise, ask away.
Byron.