I have to admit that currently one of my biggest difficulties is reconciling the contradictory information from different websites regarding the needs of different fish species. I can see that the Seriously Fish site offers, for example, a source for the Guppy information, but it's a German periodical article from 1859. I can't access the source, so I can't be critical of it. How do they know this is a good range for Guppies? How much have guppies' needs changed since then, since the fish I can buy from my LFS are probably quite different to the fish that Peters observed in the wild or wherever he observed them in 1859? Of course most sites seem worse; they either don't offer any information about where they get their data, or else they don't even mention water hardness requirements for different fish. I don't know how the SF info relates to the very contradictory information from the Aquarium Industries site I mentioned, which is the site run by the company that supplies all the potential places where I will get my fish. I'll try contacting them, but for someone who really wants to try to avoid making mistakes, and who is coming from an entirely naïve perspective in the fish-keeping world, it's all quite confusing and frustrating. One site will say that I can have some livebearers with some catfish (certainly not most), whilst another will say I can't.
The Aquarium Industries site lists guppies as requiring higher minimum water hardness than other livebearers (according to the caresheets at https://www.aquariumindustries.com.au/fish-care/livebearers/), whilst SF lists the guppy as requiring a MUCH lower minimum water hardness than the other livebearers. According to one site, I could now get Mollies, Platys and Swordtails, but not Guppies as they need harder water. According to the other site, guppies are the only livebearers that I currently could get with my 12dH GH, and the others all require harder water.
I feel compelled to simply try to raise the GH to the highest minimum from both sites for all livebearers, but it is frustrating not really knowing for sure, particularly when some of the information says that there IS a goldilocks range where I could perhaps have SOME livebearers with SOME other fish. Ho hum. Maybe I need to set up another tank with other fish species one day......
Seriously Fish is owned and run by biologists and ichthyologists. All professional ichthyologists regard the data there as reliable. The water parameter data is compiled from many sources for most species, including tests of the habitat waters.
The information you reference in your penultimate paragraph above is inaccurate. Mollies absolutely must have moderately hard or harder water, with a basic pH (high 7's minimum, around 8 fine). Some people even maintain them in brackish and full marine tanks. They don't need the salt, but my point is that they are mineral-loving fish. They will suffer and slowly weaken and die in soft or acidic water, guaranteed.
Swordtails and platies also need moderately hard and basic water, though less susceptible than mollies to slightly less hardness--but it still has to be moderately hard and basic pH.
Guppies should have the same, but this inbred species is so weak now it seem to manage with less. Wild caught fish would be very different. But how far "manage" goes when it comes to wanting healthy fish is a risk.