Welcome to the site and good luck with your starter pack.
The 25 or 30 second rule stays the same for 1 or 2 cups. The basket you use in the portafilter would be the change.
If you are making one cup, you might use a single basket (around 7g).
If you are making 2 cups at once, you will use a double (around 14g).
Some people still prefer to use a double basket even when making a single ( EG: Silvia owners like me - because the supplied single basket is cr*p), and make larger or stronger coffees.
Anyway, single shot - 30mls in 30 seconds. Double shot, 60mls in 30 seconds.
Heres my take on tamping. It compacts the coffee and hopefully sets up a firm mass for the water to pass through when making your espresso. The benefit of this is that the water does not channel or find a path of least resistance. If the water does channel, the net effect is that a portion of your ground coffee is not getting fully extracted, whilst some of your coffee is getting over extracted.
As for the expansion, I couldnt really say. All I know is that after a good shot in my machine, the puck is firm, has no pool of water on top when I release the portafilter and the impression of the shower screen screw is visible. Also bangs out in one hit and stays in one piece.
Hope that helps.