Man! Those character designs are quality!
Really great, really expressive, and really nice lines!
The sense of motion is great, and the colors are vibrant, nice, and go together well.
I think the flat-shading is really nice, and works well... however, as others have said, I think the soft-shading looks out of place. At least on the characters, themselves. The soft shading for the lighting effects on the ceiling work nicely, I'd say.
Really hope you decide to animate this at some point. Hell, if you need anybody to do some in-betweens, I'll be willing to volunteer my time and help you out. That's just how impressed I am!