Goldfish - their actual sizes ( 10-14 inches, sometimes more ) actual lifespans when not kept by idiots or small children ( 30 years at least ) and filtration requirements ( External is really needed to be honest ) One of my LFS' now has a large sign on the goldfish tanks stating that they should not be kept in less than 80 litres ( still nowhere near enough but far better than a bowl or 10 gal and is a very good start nonetheless ) I saw the guy also advising a couple not to have a goldfish , which is what they came in for, and instead showed them the small tropicals which were more suitable for their small tank. I was really quite impressed.
Bettas - If they can't be bothered to make space for a 5 gallon on a desk, sideboard or small table, then quite frankly, don't buy fish. There is also NO excuse for not having a filter. Absolutely none whatsoever. If anyone mentions current and how bettas don't like it, SPONGE FIILTERS do not create a current just a few bubbles. Therefore, there is still no excuse not to have one. Same with a heater. A warm sitting room will not always give a stable temperature, a heater will.
Plecs - They are rubbish at "cleaning" algae off the glass. And if the tank has very little algae then they don't need a plec, they need a razorblade or an algae scraper. A fair few of the commonly available bog standard plecs also get huge. The amount of times I've seen people keeping plecs to "clean the glass" when there isn't anything to clean let alone eat, is just saddening.
All of the above will obviously need to be said with less blunt force and more tact . There's a reason I've never gone for a job selling fish myself, and blunt to the point info delivery is why.
Perhaps suggest to management about creating a leaflet or caresheet detailing how to cycle a tank. Offer to write these up yourself if nessecary. Perhaps also some caresheets for the most commonly bought species.
Info on stocking levels, and which fish are likely to be agressive if kept incorrectly ( like barbs being kept in ones and twos when they need six or more ) or eat others when grown ( like angels eating neons for example )