Regular employees are really the only ones who can make a difference in the day to day operation of the store - while some fish are completely unsuitable (iridescent sharks and pacus come immediately to mind) most fish that Petsmart sells can be housed by ordinary aquarists, as long as they have the resources and the knowledge of how to do so. The more experienced and knowledgeable the employees, the better advice they'll have to offer - which is one reason people go to mom and pop stores in the first place. Places that have discus and corals (unlike Petsmart) are probably run by people who know what they are doing.
Few people see the handful of choice fish they want and build their aquariums around those fish. They usually have the aquarium already, and now they want to cram the fish they like into it. Chances are they like a lot of fish...which is why so many people end up keeping multiple aquariums. The alternative is the jammed community tank, which many people have.
I can't really think of any situation except for growing out fry, where it would be ok to have multiple oscars in a 20 gallon tank. Well fed oscars with ample water changes grow extremely quickly and will stunt in no time in such a tank. Oscars aren't exactly easy fish to unload either, being as common as convicts. If the plan is to keep them long term, one can only devote so many 4' tanks to oscars.
One issue here is that few people are actually completely oblivious to the fact that their tanks are grossly overstocked. Stocked with incompatible species is another issue, however unless they truly believe in the "fish only grows to the size of its tank" BS (and I know there are people who honestly do) they generally have an inkling that they are overstocked, and don't want anyone mentioning it. "The tank is way overfiltered" or, "it's only temporary" is often times put out there to hopefully shield themselves from criticism.
So when you tell someone what they're doing wrong, I'd say chances are they already sort of know and it's up to them to choose what's right. You can say something, but don't expect it to be well received.
For example, feeding oscars goldfish. Probably reason #2 for oscars being so often sickly after the #1 reason (being kept in too small of a tank). How many of the sorts of people who do this, are going to stop after you tell them all the reason why they should? Thiaminase, parasites, HLLE, poor nutrition...are likely to go in one ear and out the other.