The idea sounds great. I have a question though (and two suggestions.)
The question is : During a Fission move, for example, when the Nightrider
moves away from the Rook+Nightrider, is it limited to a single step ?
Update: I looked at the position you posted and found my answer, but is the position legal?
(After reconsidering, Black's last move was probably Rxb7 +.)
The two suggestions :
1. Replace the Nightrider with a Rose. The Rose strikes me, personally, as
a more natural extension of the Knight.
2. Compounds of two pieces, instead of being restricted to one step, they
can be absolute halflings. This has two implications :
a. a Halfling Rose is not how Betza defines it, but rather how he
defines normal halflings. If the Rose, along its normal line of
movement, must stop at a certain square (because of edges or
because of itself, not counting other pieces,) then a Halfling
Rose moves half that distance rounded up.
b. a King may not be involved in triple combounds. Double compounds
are okay. (Retaining some of Fusion Chess's flavour.)