Love the idea, so do you include hammer throwing Kappa and a boom-boom subspecies?
What about wings?
Basically, in Shroompunk, hammers replace swords as the most common noble/civilian weapons for kappa and for humans... because kappa are both kith's second most common enemy. Kappa knights can indeed belong to the Brotherhood of the Hammer, which gives them a soulknife-like ability to spawn and throw a limitless number of hammers. Boom-Boom and the Koopalings wouldn't be represented as a separate subrace, just as expressions of higher-level kappa class options.
Wings are an option, at about the same level everyone else is flying around anyway, but are incompatible with the heavier, spiked shell upgrades most higher-level kappa take.