I might be misunderstanding the question -- but if you're wondering how geometry created by the XParticles generator can have collisions that result in realistic rotations, then I believe you're going to have to use the C4D Dynamics instead of the XParticles one:
Put a C4D dynamics tag on the XParticles generator object.
Then in the tag's Collision tab, change the Individual Elements parameter to "Top Level"
Then you might have to change the Shape parameter too.
Hope that helps!
(And if anyone knows of a way for the XParticles Dynamics to do this -- please shar