Manufacturers tend to stick to one seat post diameter... if Cannondales are all 27.2 chances are that's what you need. However, you can get 26.8mm seat posts so if you're confident that's what you need, then go for it! You can always use a bit of coke can or something to shim it out. It's not like the seatpost will be used for much once slammed!
If the OD of the seatpost is 30.8 a 31.8 might be too sloppy. Again it depends on the confidence in your measurements! This ought to also be a standard size for all Cannondales.
BB size is axle length and shell width, neither of these have anything to do with the circumference of the shell. The width will be either 68 or 73mm and the axle length depends on the crank setup but will need to be somewhere between 118 and 128mm. You'll probably be safe with something like a 122.5mm axle but it all depends on setup.
Edit: Sheldon Brown confirms that all Cannondales use 27.2mm posts.