Is it necessary to add fuse protection (at the inputs) when using multiple DC-DC converters?
The answer depends on what you are trying to accomplish. The fuse should help protect the battery from driving a short circuit and protect the harness wiring, both of which could cause a fire.

At a minimum, a single fuse on the +28V coming from the battery before it branches out to the individual feeders is sufficient. 

Branch fuses, as in your schematic, still achieves the goal of protecting the battery and wiring while providing additional redundancy since one blown fuse won't take down the other branches.

Although the DC/DC converters may have 'continuous short' protection, there are other reasons to have the fuse (wiring and battery protection).