And they gave me extra! I asked for 6 but said if that only left 1 or 2 I'd take them, we thought there was only 2 left, ended up being 4. So I came home with 10 Corydoras pygmaeus but only charged for 8. I thought that was a pretty good deal! I have enough to try a few in my 5g again if I want to. I'm a little worried about overloading the bioload in there though.
Anyway, they're currently drip-acclimating to the 20g. I think i'll QT them using some water from the 20g, I can't decide if I should just stick them straight into the 20g though. They're from the same batch that I got cories from a few weeks ago, so I imagine they're the heartiest, disease free ones, but that doesn't rule out bringing disease IN to the tank and other fish. So I suppose I just answered my own question :-P I'll see how many survive QT (hopefully all!) before deciding whether to try some in my 5g again. It'll probably depend a lot on whether that tank is still stable, and how much the plants have grown in!
Pics of the new additions to come...