That's a really interesting system, with lots of configuration options. Good find, Darwin!
My guess, however, is that there is only one
size of ball, and that it is 2-5/16"... and it isn't a common ball-on-a-stud so you can't change it.
Scamp chose to use a 2" ball, which is perfectly adequate for the purpose, but not commonly used (heck, maybe not used by
anybody else) for in-the-truck-bed hitches.
If there is this compatibility issue (i.e. no way to couple a Scamp 19' to the Firehawk hitch ball), then there's also a fix:
change the coupler on the Scamp. Again, there's nothing wrong with the stock one, it just isn't the common size choice.
Of course there's also another way:
build your own ball mount (with a 2" ball, or better, a ball mount with common 1" hole to use any size ball), and make it removable like the Firehawk.
Height is worth considering, too. The Firehawk II mounts the ball surprisingly high for this style of hitch; usually the base of the ball is at or near the bed floor. The Scamp is set up assuming the ball is quite a bit higher than the floor, as well, so I think it's a good potential match. The gooseneck trailers I've seen with ball couplers usually have the coupler on an adjustable leg, to suit the ball height of the particular truck to be used; however, the Scamp coupler is in a fixed position.