I suppose, this approach would work with ballasting too? Though I guess I've found an excuse to put off my own ballasting for another week . . .
Yes, with care to avoid ballast getting into the crown of the clump.
Primo clumps (recommended) are packed as all of a single size - although there are a couple of lengths/heights in the range so little scope to relrepresent spreading vegetation.
Games Workshop packaging has several heights and widths in one pack plus a few longer lengths of thin width which is a good starting point for grass in cracks.
Both products benefit from trimming around the sides to remove stragglers (fibres which tend to lie flat).