The underlying formula for a sliding frictionless bead was V = sqrt(2*h*g) where g=gravity acceleration, h=vertical height dropped so far, V=speed. For a ball (or other rotationally symmetric) rolling object with zero slip, a constant fraction of the energy is in rotational, and a constant fraction in translational, energy. Hence V=const*sqrt(2*h*g). This is the same formula, just with an altered value of g. Therefore, I conclude the brachistochrone, isochrone, etc will have unaltered (still cycloid) shape, although of course you want for the center of mass of the ball to move along the cycloid, not the contact point of ball with track -- hence track would need to be a "1-radius-expanded" version of cycloid. It is unnecessary to make balls have foamy outside and dense inside to reduce their moment of inertia. Any MoI is fine. Galileo was not aware of brachist. or iso property of cycloid, but he was aware of, or at least conjectured, cycloid as best possible shape for thin beam arch (constant beam width) -- which I have discussed here, the cycloid has uniform compressive stress but not uniform bending stress. Apparently Galileo invented the cycloid. The notion of "uniform stress" is something that can be defined without needing calculus -- probably for somebody as ingenious as Galileo, pure Euclidean geometry would suffice. (Archimedes similarly computed area & volume of sphere without needing calculus.)