I agree with Jeremy that nightrider is more valuable than rook in this
game. May be its value is somewhere between rook and cardinal:
* rook - 5 pawns
* nightrider - 6 pawns
* cardinal - 7 pawns.
So, moving all nightrider-combined pieces one class up will probably
improve this game. In any case, this will avoid that most of the game
start with rook->nightrider pocketing as of now. Certainly, some
play-testing is needed to see if really there are situations where you
will prefer nightrider over cardinal.
Other possible alternatives:
1) Add a new piece class between classes 3 and 4 and move nightrider and
SuperRook there.
2) Remove nightrider-combined pieces completely to simplify the game and
make it more strategic.