Updated with dynamic controls assignment, which should let you play with any combination of (up to 3) keyboard and gamepad players (up to a total of 6). Whatever input you use first automatically maps to player 1, the next to player 2, the next to player 3 (spawned on demand), and so on.
Please let me know how this works for you. :)
Also, I'm digging "Capture the Marble" - I've been itching to try getting one of those swirly glass marbles in. Initially I was thinking of it as a hazard, but now I'm considering "Marble Soccer" or other uses... Stay tuned for further experiments over the next month!