Hi,
IME cories will dig into the substrate for food, but not like drilling a well...
They will point their snout/barbels at the food and shake vigorously with their head to pull out worms and such..
So, while they don't go deep, they can easily injure themselves with a sharp substrate...
You will see that their barbels are much shorter than usual if your substrate is too rough..
I can't think of many commercial that are bad this way unless crushed coral?
I have used flourite with no issues...
2-3" based on species but most SA cories are not that large..
They like company fer sure, so 6-8 is a min school in a 55.
12-15 would be nice based on how many/what type of other inhabitants, filtration, etc.
Remember that they need to be FED not just eating fish poop or some nonsense...I fed mine blackworms at times and other foods...
Most cories remain somewhat active in the tank (many more at night) and school well. Be careful of bottom dwellers/breeders (Apistos) that may not appreciate cories....
Some of the more common cories will easily breed and lay eggs in a community tank, but so far I have not seen any youngt survive, but eggs plastered all over....
You can also mix species, say 4-5 of 2 or 3 species, as for the MOST part they get along very well. I say MOST part as I am not familiar with every cory species