Any question answered...

How would I know if a seatpost is suitable for a carbon-railed saddle? What are the characteristics I should look for?

Depends which carbon rails. 7mm round ones fit most seat posts, some people suggest you should avoid short rail clamps, excessive overhang and sharp edges, but you should already be avoiding those things regardless of rail material. Many carbon rails are oval 7mm×9mm, which will be fine with some clamp designs but either need special clamping parts or won't work at all with others.