Best I can tell, salt seems to have a slight beneficial effect if - and only if - guppies (or other fish) are stressed by something.
It is probably most beneficial to inexperienced aquarists, who haven't yet mastered cycles and water changes; as salt helps lessen the effect of nitrites and other toxins. And for fish undergoing repeated stresses and parameter changes, like those passing through a LFS.
It can also sometimes provide aid to a fish suffering from an undiagnosed ailment, or one that is weak due to poor genetics or prior injury.
Perhaps in very soft water, salt may be useful. I've kept guppies in water with 0°GH and 8°KH, at pH down to 6.5, with no issues. And if water hardness is a problem, assuming plants are present, then that would be better solved by boosting the hardness rather than salinity. Our tanks are now generally kept at a minimum of 2°GH.
Back when I regularly used salt, I think I started with "Doc Wellfish" brand. It appeared to be just rock salt. So when I ran out, I switched to rock salt, and it seemed to work the same. Table salt is not the same though, as it contains an aluminum-based anticaking agent which is probably better off avoided.
Note that neon tetras and otos are very sensitive to rapid changes in salinity. Cardinals probably are too. Drip acclimation is highly recommended when moving them into an environment with different salinity.