There are a few general points to be remembered when considering pH.
First, pH is closely connected to the GH/KH, and the GH is actually more significant on fish.
Second, some fish can adjust to a certain extent, some fish cannot; or if they do, it is short-term.
Third, the fact that any fish is alive in the store tank for the limited time it is there does not mean the fish is not experiencing stress from their water, and will in yours too.
A comment on the pH ranges, yes, you will find variation with this as with almost any aspect of this hobby. One has to consider the source of the information in order to determine its reliability, and these days, anyone can set up a web site and say whatever with no actual knowledge behind it.
We have fish profiles here, and the ranges for water parameters (GH, pH and temperature) are reliable because they are the consensus of knowledgeable ichthyologists and experienced hobbyists. I wrote most of the profiles, and if i ever find a contradictory value from a reliable source, I mention it so one has all the facts.
To your case, you will have no trouble with any soft water fish, perhaps with a few exceptions and those being wild caught. The pH will tend to lower, and with your GH (and presumably low KH) I would expect your tanks to run just below 7 when they are established.
Final comment on short-term adjustment of fish to parameters outside their preference. These fish usually die sooner, i.e., they do not live to their normal lifespan. For example, cardinal tetra rarely live more than 3-5 years when kept in anything other than very soft water; in soft water, they can live more than 10 years. During the four or five years in harder water, everything appears fine. But the fish will die. Usually it is calcium blockage in the kidneys, something you cannot see without dissection after death. I wrote more on this in a recent thread. But the point is, that all fish evolved naturally to live in very specific water parameters, and it is simply not feasible that we are going to change thousands of years of natural evolution just because we may want to.