Correct frame size, will 56 and 58cm work?
You have a few things to consider. First a Cannondale 56cm geometry is not identical to a Giant, Specialized or likely any other brand. Each brands designer has a specific goal in mind when they design a frame. So you can make a generalized statement that you may usually ride a 56cm it is not a hard and fast rule. One thing to consider is with the advent of compact frame designs the dimensions become virtual. This means that measuring the frame will yield a number other than 56cm. If the frame isn't marked for size you have to verify manufacturers dimensions to determine size or ask the owner how tall they are.
The most important thing to remember is not size but fit. The most critical is stand over height. If you can't comfortably straddle the frame it's too big. It also has to be comfortable, if you haven't ridden a drop bar bike before it will likely feel awkward for the first few miles. I would test ride a few 56s but don't overlook a 54 or two for test rides. Similar to buying a house or car try not to fall in love with the first one you ride.