The way bike seats fit to the seat post is pretty standard. If the seat has rails for the mount, you can be fairly certain it will fit most any seat post. As for size of the seat post going into the tube. That is another matter. But if you'd looked at Fuji's site, you'll see they show it as 30.9mm. Can you cram a 31.6 in there.... maybe but why will you want to do that when you can get the correct size?

Any rear derailleur will likely fit. But won't necessarily work with the cassette and crankset and shifters you have. Why do you not already have these components? Did you only buy a frame?

No one here, including me can tell you anything for certain without knowing exactly what you have currently and what your goals are.