In short, no. Seatposts are an area where bike manufacturers seek to make gains in weight, aerodynamics and comfort – so there are a whole range of different types that come with different bikes.
If your bike has D-shaped or aero seatpost, like the Specialized Pavé, it is unlikely you would be able to upgrade it without going back to the specific manufacturer. Some bikes, such as the Giant TCR, even have integrated seatposts that cannot be replaced.
If it has a round seatpost, however, you are likely to have far more options – but diameter becomes all-important. The two standard diameters are 27.2mm and 31.6mm and all of the seatposts featured in the guide will fit one of those two sizes, while some have more options in between.
Some bikes even go as narrow as 25.4mm, Cannondale being a case in point. A skinnier post will provide a bit more flex, and therefore a slightly more comfortable ride, while a fatter one will be more robust and add stiffness for additional power transfer.
The length of the seatpost will also need to be considered. If you’re on a frame that’s too small or you have extremely long legs and need a lot of seatpost sticking out of the frame, it’s something to keep in mind. Conversely, if you’ve got short legs and ride an XS frame, your seatpost may bottom out and still be too high. Cutting down a seatpost is no big deal – just remember to measure twice and if it's carbo use an appropriate blade and wear a mask.
Most of the best road bike saddles use a two-rail system, however not all two-rail saddles are compatible with all seatposts. Saddles with alloy rails will have 7mm, round rails, while most carbon-railed saddles have 7x9mm oval rails.
There are also a few saddle/seatpost brands which use a proprietary clamp and rail design; although these have largely died out because most people don’t like to be locked into a specific system.