This will primarily repeat what other members have said, but I may be able to explain things using different words which sometimes helps in understanding.
Freshwater fish species have evolved to function in fairly specific water parameters (and the whole environment actually). Their physiology operates effectively within the specific range, and outside this range the fish has more difficulty carrying out basic every-day life processes. This wekaens them, making them susceptible to disease they would normally be able to fight off, and it leads to a shorter lifespan because the fish just cannot function well.
Livebearers are fish that must have a certain level of dissolved calcium and magnesium in their water; the GH is the measure of these hard minerals. The fish cannot assimilate sufficient minerals from feeding; the dissolved mienrals must be in their environment water which is continually entering the fish via osmosis through every cell, into the bloodstream and internal organs. The water must therefore have a minimum level of these minerals.
By contrast, fish evolved to live in soft water do not need these minerals, and if the minerals are present in the water being pulled into the fish continually, the minerals must be filtered out by the kidneys. As these species are not designed to do this, the calcium forms blockages in the kidneys, and here again the fish are weakened until they die. This weakening causes stress and other internal problems, so they too are more susceptible to disease.
There is no middle road, generally speaking. Livebearers and some other fish need moderately hard (or harder) water. Some (most) soft water species need soft or very soft water. Fish from these two groups cannot exist in the same water. There are a number of species that are somewhat in the middle; theswe are by evolution softer water species but because of their specific natural habitat waters they are able to function quitee well in water that is somewhere in the middle.
Another thing to keep in mind are the necessary partial water changes that keep an aquarium healthy for the fish. Regular once a week changes of 50-70% of the tank volume is minimum. If you canuse the source water as is, this is not at all difficult to manage. But if you must adjust the parameters, which must always be done outside the aquarium, this is an extra burden. And especially when/if emergency water changes may be called for, and they will be. Selecting fish suited to your source water makes your life and that of the fish much safer and easier.