Last year we WERE going to have a runway, but due to some sort of hotel glitch or something like that, the runway didn't happen. In fact, I didn't even hear about the runway being nixed until just before the convention.
Traditionally, walk-ons at Kumoricon can either supply their own music, or walk onstage using whatever generic music the sound guy puts on. So, yes, they have always been allowed to choose their own, and many do since they know what's appropriate for their character. Also traditionally, all walk-ons crossed the stage prior to the start of the skits.
I'm not sure whether we could coordinate alternating walk-ons and skits. The exact numbers of presenters is extremely variable. Lots of people sign up, some don't bother to show up at all, and then others sign on at the last minute. Last year, we had 5 skits not show up - yet got a TON of additional costumes / walk-ons.
This year we are hoping to have a runway so as long as all goes well, one ought to be present. It would be even more fun if there existed a convention space that had the equivalent of a Kabuki theater's stage - that would really be a lot of fun (in addition to there being runways in center stage, they have runways on stage left and stage right, cool trap doors and rising platforms). We have also decided that we're going to have the walk-ons present their costumes during the "half-time" (the time during which the judges tally the scores to decide the winning skits).
by the way, this year we have 2 Cosplay Coordinators (yes, Kumoricon is getting big enough to need this). I'm running the costuming half and Cosplay_Manda is running the skits. We ARE working together so if you find either one of us we should be able to answer all questions.