The first time i ever successfully breed cory's it just happened, i had 2 albino cory's which i got from my
lfs and they breed about a 6 months after i got them. After that i got hooked. They do say that cory's should be in big groups but like i said earlier i have done it with just a male and female.
I will admit that bigger groups did work very well, i went out and brought 6 females and 18 males and that worked fantastically. I found that they always started to breed in the mornings, the female would clean various sites around the tank ie.. glass, plants, rock, side of filter etc... She would then produce some eggs and hold them with her pelvic fins, she would then go to the males one by one and then throw the eggs on what ever service she had cleaned. This would go on for several hours until she has laid up 200 eggs. it can sometimes go on for days.
I have breed cory's in various conditions ie......
A community tank 36x12x12 gravel substrate, plants etc..
A QT Tank 24x12x12 No Substrate ( i think i was just lucky)
Then i had a cory tank made it was 60 long x 24 wide x 10 deep with a sand substrate and lots of plant and all that was in there was cory's.
I also found that they breed after an introduction of some cooler water ie.. water change.
Some good cory's to start with are
Albino cory, Bronze cory, Peppered cory.
HTH