KC-
First off, congratulations on your interest in FG. I think you will find it to be the most satisfying bike riding experience of you life.
I think that I can safely get a 3 tooth difference from 15 to 18 on my Pista (48T front) without adjusting the chain length but I'm not sure right now. I will have to check when I get home. There is something that you should consider if you are going to run two fixed cogs:
A lot of Track hubs are inexplicably designed to accept a fixed cog on one side and a freewheel on the other(or two freewheels but not two fixed cogs), including the hub on the stock wheel of a Bianchi Pista. These are commonly referred to fixed/free flip/flop hubs. To run the setup you are speaking of you would need a fixed/fixed flip/flop hub which limits your options. You can go with Phil Wood or Shimano Dura Ace and break the bank or Surly which is significantly cheaper and the people in this forum will praise them up and down the street.
In any case you will need to purchase a rear wheel which can cost some $.
Or if your riding is going to be really low impact you can have a bum bike fixed setup(fixed cog lotsa locktite and a BB lockring) on the freewheel side of the stock Bianchi wheel. This may be dangerous and since you are a beginner I wouldn't recommend... although some would say you wouldn't have a problem.
Good Luck!
Neal