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.

You'll really want to look at the specific geometry of the bike, rather than the seat-tube size. As stated in the other answers, standover height is important, and should be shorter than your inseam by 1-2cm. Besides that, Stack and Reach are probably the second most important measurements to consider.

If you can't try on a bike in person for pandemic reasons, there are many ways of calculating your "ideal" stack and reach by measuring various parts of your body and punching them into online calculators. I'll leave a couple of links below.

https://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp https://www.bikeexchange.com/promo/bike-size-calculator

As many have pointed out in comments and answers, bike sizing is not an absolute but a “fit window,” where it is possible more than one frame size could work for a particular rider. That said, given you dimensions, the fact my height and inseam are within a cm of yours, and the fact I have trialed risen a large number of brands and sizes I can say with a high degree of confidence that you will not be happy with a 58cm Cannondale road frame.

Their bikes tend to fit low and long, the 58 will be too long with out resorting to extreme measures (e.g., ultra short stem). It might work as a flat bar road bike, as you typically want a longer top tube with a flat bar setup - but given that you are a beginner I doubt you want to undertake that costly change.