first petco IME is terrible, petsmart the better of the 2.
Second research on here and elsewhere please dont take my word for it, I would prefer you took the time to verify what I advise.
Third, your biggest concern right now is ammonia and nitrites, those are what will kill your fish in small amounts, nitrates will always be present to some degree (once the cycle has begun in earnest) and are fatal in high doses, you really shouldnt let you nitrates go above 20ppm but 40ppm is still considered safe by most test kits. Ph can kill them if they come from a store that has a ph varying .2 difference and you do not acclimate properly, I always test the water in the bags so I know how to properly acclimate to my tanks before adding new additions. Once you have your tank established the ph will remain fairly the same as long as you do proper maintenance and unless trying to breed certain species you should avoid adjusting it at all.