Hi all,
Corydoras pygmaeus is fairly easy to find and fairly cheap. C. habrosus is a bit more uncommon, and you don't see C. hastatus for sale very often at all. Corydoras pygmaeus & C. hastatus are both really smart little fish, but C. hastatus is definitely more of a "looker". I've never kept C. habrosus.
It may be due to the tank they are in, but I see the pygmaeus much more frequently than I see the hastatus, which tend to behave more like a bigger cory and be bottom orientated. The pygmaeus are all over the tank as a shoal, but do occasionally rest on leaves in 1 and 2's.
If you have a look on PC, "Apistomaster" has some threads, on all 3 dwarfs, where he talks about how he breeds them as a colony.
cheers Darrel