A quick update.
Went to an LFS today and got another two pygmy cories to make 6, intend to keep the lights off for now to help them settle down.
Bad news about the upgrade though. I found when doing a w/c yesterday that my tank isn't quite level, which surely means that my shelves aren't coping too well with the weight of the tank. Even if it's only an extra 7L or so, I'm not sure if I should risk it. I guess it's goodbye to a larger tank unless I can find another (stronger) surface to put the tank on, which I don't currently have and have no space for another piece of apropriate furniture
EDIT:
Just had a thought - if a new tank had a slightly larger volume but a larger area (the current tank is a little taller than it is wide and deep), would the weight be distributed better, making a newer tank more viable? Just wondering if the weight would be distributed better than it is now, meaning less weight in one place for the shelves to cope with, if that makes sensce. I just don't want to push it though