With a Bianchi Pista, you can assume standard ISO 1.370 x 24 tpi threading for a freewheel, and any freewheel from 13T that you would buy will come that way. As far as going with a smaller chainring, that would simply let you get by with using a chain tool and a 5 mm allen key to switch the ring out, rather than remove and replace the freewheel. There are arguments that larger diameter chainrings and freewheels are more efficient, though, so yeah, go to an 18T, or even larger if you wish.

Apparently the FSA Tempo Track crankset has no published specs - but in all likelihood it's either a 130 mm bcd (bolt circle diameter) crank or a 110 mm bcd crank. If you consult the late Sheldon Brown, he provided us with a great cheat sheet to measure and find out. At some point you may wish to have a variety of rings and rear freewheels to experiment with.

Just as a fun historical note, in his 1936 Every Cyclist's Handbook British author F.J. Camm recommended 60-65 gear inches for single-geared road work.