The pH is important, but GH is the more crucial for fish, provided the pH is not extreme. And it is not extreme here. Also, now we know the GH, I would expect over time the pH will tend to naturally lower as the organics break down. Water chemistry is a very complicated issue, and many make the mistake of trying to adjust it at the beginning, with terrible consequences. It is better to know the parameters, and let the tank establish the biological system. It will remain much more stable this way. My tanks have been running for years, some of them six or more, some two or three years if I rebuild them for different fish. I only test pH sporadically because I know it is stable and will remain so provided I continue my water changes. Over several years, having kept records of all tests, the pH in each tank has never varied by more than two decimal points (example 6.2 to 6.0 and back). Each tank can be different as each biological system is unique, notwithstanding the same water, same maintenance, same fish load, and plants, but the different pH levels within each tank have remained stable. Same for nitrate which is in the 0 to 5 ppm range.
As an aquarium establishes over the first few months, the organics accumulate in the substrate and the decomposition of these produce CO2 for the plants; CO2 is also occurring from respiration of fish, plants and some bacteria species, but the CO2 from the substrate is the major source. CO2 produces carbonic acid, which acidifies the water and the pH lowers accordingly. I let this go where it may knowing it will stabilize. I select fish suited to the GH (mine is zero) and an acidic pH. Some fish have a wider tolerance than others, but if the system is stable this is less of a problem.
The hatchetfish best suited to this tank are the smaller species, all in the genus Carnegiella. The species in Gasteropelecus and Thoracocharax are a tad larger, and also more active, and like to remain below the surface rather than at the surface. I have or have had species from all three genera, and the latter two really do best in a deeper tank than thee 40g breeder. My Carnegiella strigata, C. marthae, C. schereri and C. myersi did very well in my 40g tank. Carnegiella strigata, the Marble Hatchetfish, is probably the best of these; the other three can be very sensitive.
Bettas are not community fish; while one may seem to manage in a tank with other fish, this often will end very suddenly, and along the way there are undoubtedly issues for the fish--the Betta and the other species--that we do not readily see until it is too late, and meanwhile the poor fish have been suffering. And bettas and gourami do not mix well at all. Gourami species frequently do not get along with each other. Males are territorial, some very much so. I am not much on "centerpiece" fish as they usually bring problems especially in small tanks like a 40g at 36 inches in length. If you decide on hatchetfish, a dwarf cichlid would make a better tankmate than gourami. A solitary Bolivian Ram is one.