To your initial question, as some others have noted, not all tetras can be housed in the same tank, and not all barbs either. And temperature can factor in. The GH of 75 is presumably 75 ppm, so you are dealing with soft/very soft water which gives you considerable options from the aspect of parameters. All of the tetras, and pretty much all the barbs, plus most of the danios, rasboras, loaches, catfish (like cories)... literally a whole world at your fingertips.
Compatibility is often misunderstood. Parameters are key, then there is the habitat environment--flowing water or still water, lots of plants/cover, sand substrate or gravel, rocks, or wood, or both...all these impact the health of the fish.
