Actually, I know an Emperial gallon (Vs. a US gallon) is larger, though I don't know by how much. I'm sure there was a logical reason (like we just had to change it cause we had a revolution... or actual trade issues...heh) but I don't know what they are.
I've always wanted corydoras catfish. I think they would go good with your stock because they're bottom dwellers and a lot of your fish are middle-top dwellers. There's lots of different kinds of cories too.