1. In all elements where mass and density (and similar measurements) are mentioned--e.g., <rigid_body>, <shape>-- each should reference the "About Physical Units" section.
2. The "About Physical Units" section says that, "If needed, units should be taken from the “base” of the COLLADA document". However, it doesn't say how you'd do something different (other than using <asset> to set the distance). For example, would one use <extra> to specify that other units are to be used? (for time, angle, mass...)