All of my cories except the peppered one are very active and don't hide. The peppered ones in my 29 tank and Stone's 46 both hide all the time. It's to much the same for it to not be just how they are - at least for us, we think they just like to hide.
I have 9 pygmy cories in my 10 gallon witn a halfmoon betta and they are happy and active. They are TINY so in a 20 gallon or bigger you'd never see them. I've seen people on anotehr forum recommend keeping 20+ in a 10 gallon and they will be happy and healthy and live just fine in it. Like I said - they are TINY. Personally unless I had a huge school of them, I won't keep them in anything bigger than a 10 gallon they are already like trying to find an otto in a heavily planted 90 gallon in my 10, I like to actually see them.