First, I consider all species relative to their habitat water parameters. No one has ever provided evidence that fish do not do well in water comparable to their natural environment. Regardless of commercial breeding, this does hold true. This does not mean some species cannot be healthy outside their preference parameters, just that using their habitat parameters will be safe.
Second, within the above, I group fish into very soft, soft (usually consider these the same frankly), or moderately hard (or harder) water. The "middle ground" between these can be suitable for some soft water species and some harder water species, within reason. This is a generalization obviously, but it holds.
Guppies will be healthy in moderately hard or harder water, because that is in their evolution. This does not mean they cannot be healthy (or manage at any rate) in somewhat softer water. I had a female guppy acquired by accident (came in the bag with other fish) that lived for months in my soft water tank. However, if I were to keep guppies or any of the commonly-available livebearers, I would do so in moderately hard water and not in a soft or very soft water tank. In the 1980's I had a tank of black mollies and I used a calcareous substrate to maintain the GH and pH suited to mollies. Mollies are very different from guppies, but the point is that if I intend keeping "x" fish I will provide what it has evolved to require, regardless.
Guppies acquired from stores today are genetically compromised, according to most reports I've seen. Many commercially-raised fish are suffering the same fate.
Concerning Corydoras pygmaeus, this is a soft water fish. It occurs in blackwater as well as clear water, but this water is very soft and on the acidic side. There is nothing wrong with keeping it with guppies so far as the two species' behaviours, etc, but they do not share similar water parameters naturally. There is probably some overlap however; this is the general view of most sources. I would want to provide parameters closer to the cories simply because they are more likely to be affected long-term, especially if wild caught [not sure if this species is being commercially raised somewhere or not]. As I said above, I always aim to provide water as close to the habitat as I can, and I do not differentiate between wild/commercial fish in doing this. It has worked for me, because I only keep soft and very soft water species so I am not going to be pushing the envelope, so to speak. Following this is one more step to trouble-free tanks.