Depending upon the livebearer in question, it is very difficult to determine exactly what is best for them because you can't know where they came from. There are some mollies, for example, that really do come from slightly brackish waters, yet not all do, and most have been bred in aquariums for so long that it is not necessary (trying not to get anyone started on the "salting of the molly" red-hot issue... just an example!)
I agree that if they are thriving and doing that thing they do, with viable fry, then you are in good shape. I am keeping some in my African tank that is going to someday be home to some of Voodoo's shellies (
) and that tank has calciferous rock, buffering sand substrate and calcium carbonate in the filters, because the shelldweller cichlids really do better with hard, alkaline water. However, when the shellies arrive I do plan to acclimate my little fry machines to one of my community tanks, with straight tap water.
Voodoo - I am jealous of the Endler's! The only time I saw any around here they were in with the guppies in a huge tank, and there were too few to school, so the chap could not catch any for me. I need to join a local club, but right now the only ones are salty that I can find. Someday, in my spare time....