There are a couple of issues mentioned in this thread that could do with some clarification.
GH is the most important parameter (of the three, GH, KH and pH) for fish. No species of fish can adapt to significant deviations from the GH of its native waters. There are some species that have a wider tolerance, within reason. Stores rarely bother with this because they do not intend keeping the fish for very long, and it is the permanent long-term care that does matter (to the fish). Once the GH is within an acceptable range, the pH will usually follow. The GH and KH affect the pH, which is why one should never target the pH alone without managing the GH (and KH).
Livebearers (all of them) must have moderately hard or harder water, there is no getting around this. Soft water, and/or acidic water, will weaken the fish more and more until it succumbs to some disease/issue it would normally have been able to easily handle; sometimes the inappropriate GH/pH directly cause its demise. The fish cannot live a normal life, in any sense of the term, simply because the GH is a detriment rather than a benefit. Both soft water species and hard water species have this problem, in opposite directions.
When it comes to raising the GH, it must be done correctly by adding calcium and magnesium salts dissolved in the water, and dissolved prior to the water entering the tank (to avoid shock). This can be achieved through the use of mineral salt preparations such as those for African rift lake cichlids, or through a calcareous substrate sand that will continually dissolve calcium and magnesium. Coral, shells, etc are not sufficient for this; they can work for shrimp, but not fish. And they are not adequate buffers.
Equilibrium was mentioned. I used this in two of my tanks for five years. It raises the GH but does not affect pH or KH. My tap water is zero GH/KH, and I used sufficient Equilibrium to maintain the GH of the tank water at 5 or 6 dGH. This obviously was not for livebearers, or indeed fish period, as Equilbrium is a plant additive. I contacted Seachem about Equilibrium and Replenish and they advise to use E only to benefit plants, and Replenish for fish requiring harder water. I spoke with one of their technicians, and I only know what he told me.
For livebearers the GH must be at minimum 10 dGH (180 ppm) for guppies, platies, Endlers, and possibly swordtails, though it would be better around 12 dGH or higher. Mollies must have it in the harder range. And while some "soft" water species could manage, many cannot, and all of them will be healthier in softer water. Point is, there is really no "middle ground" as such, if we care about the fish. Prepare harder water for livebearers, and leave soft water fish for a tank with softer water. Both groups will then, and only then, be more likely to have good health and a decent life.