Hey BoLS,
Kirby ::barf:: ::fart::, has stated that ICs can join the Riptide because it can have drones, making it a 'multi-model unit'.
Is this the case?
I think not. Why?
Drones are a special circumstance to the normal rules for 40k.
A commander takes 2 drones as wargear. Now this is a 3 model unit, and only one model has the IC rule. However, the Tau Codex makes an exception that the drones do not count for the purposes of joining squads. Essentially, the Commander unit 'counts as' a single model for the purposes of the IC rule.
Since this is the case, the inverse of the rule should be applied to all models taking drones as war gear, for consistency.
A Riptide that takes Missile Drones should 'count as' a 'single model unit'. No ICs can join the Riptide.