sop3882673 wrote an answer about Snow Peak Titanium Single Wall Cup 600 on June 5, 2011
To answer your second question first - the difference between the 600 and the 450 is volume. The 600 is a 600ml cup (about 21oz) and the 450 is a 450ml cup (about 14oz). The 450 is SMALLER than the 600 (and weighs about .5oz less).
Now for the first question - the smallest canister that I know of is the Jetboil Jetpower 100g canister. It's dimensions are 3.5" diameter by 2.5" tall. The dimensions of a 600 are 3.8" diameter by 4" tall. So YES, this canister will fit inside the 600. However it will NOT fit inside the 450, which has a diameter of only 3.4".