I've shortened an inexpensive carbon seatpost maybe 12 years ago. And cut off carbon steerer tubes too. Those short cutoff tube sections are really strong. The 6 nM won't hurt the seatpost with the compression from the frame clamp.

At random, I checked Ritchey's website for carbon seatposts.

Their $58 Comp 2-bolt says "Seat collar torque: not specified - follow frame/seat binder specification"
Their $99 WCS Link says "Seat collar torque: not specified - follow frame/seat binder specification"

Their $270 light Superlogic post says: "Seat collar torque: 6Nm (max) / 7Nm (max) for Ritchey steel frames" And it's description says: Made from a lightweight carbon fiber layup developed for Pro Tour teams at the highest level, the SuperLogic Link post offers more vertical compliance with no loss of lateral or torsional stiffness.