However, given the traditional inclusion of Tamil, Telugu, Kannada, and Malayalam among those scripts covered by ISCII their common grouping in Unicode, it is entirely appropriate to treat them in the same document as Devanagari et al.

Glenn,

Why? Do you consider that they have similar layout requirements to Devanagari?

Yes, similar, though not identical. But certainly related. [Based on my personal experience of implementing layout support for all of these scripts.]