Daz,
Pygmy's are pretty tough, like all corys. They will do best in softish water, but I know where your getting yours from, and they have been bred in hard water, so will be fine for your tank.
They probably need a finish substrate, and would need to be kept in a group, say 6-8.
They'll grow to maybe 3cm.
You could get 6 in a 5 gallon tank, but you would then be pretty much full. They would probably be best to go in your big tank when you get it.
Steve