I am not sure where do You get this 3* info from. Hengeli lives alongside Betta coccina, (0-10dh). Pangio semicincta was found alongside Chocolate gouramies, which live with Parosphromenus in Kalimantan Barat.
I have 8 Paros, 16 Boraras in 70 litres, and it’s not overcrowded. The amount of fish you can safely and comfortably put into the tank depends almost solely on the skill to create hideotus, broken lines of sight etc. 16 is a bit to much, I’d prefer 12, but it’s not that awfull. When they get scared they form a schooling group and they look ok.
Now the question of the water hardeness. From what I’ve read in this forum and the site itself I came to think, that the most important is the 0 KH. The varying pH of several succesful spawnings may point: KH should equal 0, an GH should be as low as it goes. But I doubt that having the GH even up to 8 ( I want it to be around 3-5) is bad. It’s just not ideal for breeding. The leafs falling into the water have some minerals. Than they are eaten by shrimps and/or get dissolved n the water. There is SOME mineral content. Low, but still. These few micro-siemens in the water conductivity are coming from somewhere.