Completely agree. I do wish stores would educate their staff better. I have an online friend who worked in a store for 20 years, and they have a training program that fish staff must take. Terrific idea.
To the corys, five is minimum in an aquarium, but where space permits, more will be better. This is a highly social fish, living in groups of hundreds in the wild, and without companions it will be stressed and that leads to health issues.
By the way, I would venture a guess that your Julii corys are likely not Corydoras julii but C. trilineatus. Throughout North America, C. trilineatus has been marketed as C. julii for years. The two species are very much alike, but you will only see true C. julii if they are wild caught fish. C. julii is quite rare in the hobby, since it occurs in rivers and areas that are generally not heavily-fished commercially and is therefore seldom exported; the few times it does appear it has probably been collected in the Rio Para which is regularly fished. The true C. julii has a spotted pattern on the head and body, and the lateral stripe is either not present or extends only midway along the body. C. julii is also somewhat smaller and more compact-looking in size than C. trilineatus. The latter has more of a reticulated pattern (as opposed to dots) on the head, and the body markings are more variable. But the head pattern is usually sufficiently distinctive to discern the species. First photo below is a C. julii, second a C. trilineatus, but keep in mind that the patterning can vary greatly from what is shown on these particular fish.
Byron.