I don't need to tell you that Riptides are amazing do I?
They are strong, tough, and mobile... They are reasonably cheap... They carry some amazing weaponry... You can equip them with excellent Support Systems (and some decent Signature Systems if you're a Farsight Commander)... They can even be threatening in Assault (in the right circumstances).
What some people don't realise is that it is also possible to bundle them together with certain Independent Characters in order to make them seem truly broken!
The problem is, some people attempt to argue that the Riptide cannot be joined by an Independent Character because it's a Monstrous Creature. Others argue that you can't join it unless the Riptide took it's drones.
These arguments are all fairly ridiculous if you look at the underlying rules clearly. So let's take a look at the definition in the Basic Rule Book on p39:
Independent Characters can join other units. They cannot, however, join vehicle squadrons (see page 77) or units that always consist of a single model (such as most vehicles and Monstrous Creatures). They can join other Independent Characters, though, to form a powerful multi-character unit!
The key point here is (paraphrasing) "They cannot join units that always consist of a single model". Try not to be distracted by the inclusion of "such as most vehicles and Monstrous Creatures" as this is really quite irrelevant. The writer is trying to give a broad example (using the word "most") of what falls into the category, but it gives us enough wiggle room to provide specific examples that do NOT fall under this banner.
Page 33 of Codex:Tau Empire provides additional information to set this whole matter straight:
Drones taken as upgrades for a unit act as additional squad members in all regards
So given that, there is enough information here to draw a definitive conclusion:
A Riptide is a UNIT that potentially CAN consist of multiple MODELS (in this case, Shielded Missile Drones, who are treated as Squad members in all regards), therefore it CAN be joined by an Independent Character, whether or not actual drones have been purchased.
This all seems really straightforward to me. So grab your buff Commander, Farsight or Shadowsun and feel free to join them to your Riptide (with or without SMDs).