For a compact frame that will require a longer seat post think about what you will be using the bike for. How important is stiffness compared to comfort? Will you climb more than sprint? What is your weight? Do you ride high cadence (above 100rpm)? If you're curious about carbon try getting on an indoor trainer and spin at 110 and have someone describe the flex they see on your carbon post, you might be surprised. For a longer post try aluminum. Light and stiff.

70kgs
cadance 105-115 training - 95-105 racing
climbing - cobblestones

should the selcof carbon monocoque 340mm be stiff/strong enough???

For climbing and based on your weight the selcof is fine. It will certainly dampen those cobblestones far more than any alu post. Based on your cadence I don't think you climb off your saddle, but if you do you can go even lighter with an axlightness saddle and post combo. Very expensive but also very light. It is more comfortable than you would think by looking at it.

don't really need that ultra light
to dangerous for the tour of flanders and paris roubaix!
my stem broke once...
i'll go for selcof+fizik arione
seems good
my bike will reach the uci 6,8 limit