grain distribution:periodic or scattered)

Yes, a little vague, but the quickest route might be to trigger the grains using your own algorithm; perhaps regular triggers via a simple metro, or [phasor~] -> [> 0.99] -> [togedge]? These could then be randomized using either [random] or [rand~]. Look also at [rslider] which allows one to specify a range, for constrained randomization.

The alternative would be to get under the hood of the [rgrano~] abstraction
and have a good ol' root around in there; after saving it under a unique name first of course, don't wanna break those example files.

Brendan