Trust me, it is not always an unlikely event that entire tank full of fish will succumb to S. vortens. Nor does it always take months for a fish to die. Death can come in at various stages, in many cases fish don't last more than a couple weeks, and sometimes it is a secondary infection that eventually causes a weakened fish to die. I have seen it all over the years, in numerous species. Entire tanks full of Tropheus sp., hundreds, sometimes thousands of $$$ of fish gone in less than 30 days. The scientific community agrees, they even mention in the paper that I linked to in my sticky on this subject that in commercial angelfish hatcheries S. vortens can cause as high as 50% mortality rates. And it doesn't take months, or years.
If you are interested in learning more about S. vortens, here's one of the best reads on the subject, and there are scores of others.
https://theses.lib.vt.edu/theses/available/etd-120399-140825/unrestricted/ANGEL.PDF
A much shorter version, that touches on most of the basics.
http://edis.ifas.ufl.edu/vm053
Do livebearers last longer than other species? Possibly, I have no idea as I don't keep livebearers, nor have I ever read any research specifically with this genus or species. What I can tell you is that the people that have done the most research in this area will tell you that there is still much they do not know. But there is lots that we do now know, and one very important thing is that this species of parasite can become systemic, which depending on various factors will often be the deciding factor on how long a fish lives, or doesn't.
For the most past S. vortens is a "cichlid" disease, and while it may be found in various other species of fish, there are numerous pathogens that could mimic this parasite - one can really only be certain when viewing the parasites under a microscope. With livebearers, the symptoms that you describe are most commonly associated with mycobacteria infections, which is very common within all of the various livebearer families.
http://www.fancyguppies.co.uk/page42.htm
Again, without a microscope hard to say with certainty. Either way Spironucleus vortens most definitely attacks different species of fish, and even different fish within the same species, differently, depending on various factors previously mentioned. This is not a one stop shop form of parasite.