It can be based on a rogue and a ninja and still be pure water. I mean apparently Fennekin is supposed to be a Mage/Witch but is Psychic type. Mages and Witches deal in Magic not Psychic powers. It would make more sense for Fennekin to be Fairy which is a more magical type than Psychic to fit the bill, and as for knights, Steel would fit better than fighting stricly because the most notable thing about a knight is their armor.
Based on some of the moves that Chespin and Quiladin have been showing off I still have not been convinced its not a Rock Type.
I have to agree with you there, a Pokemon's appearance doesn't always judge if it's pure-typed or dual-typed, and I do admit your type combination predictions (Grass/Steel, Fire/Fairy and Water/Dark) could fit the descriptions for the final evo Kalos starters on that whole "knight, mage, rogue" theme they have going on. Although I wouldn't be surprised if they remained pure-typed, I mean we did have a set of starters that all stayed pure-typed (Johto) and then we had another bunch that all ended up dual-typed in their final stages (Sinnoh). Most cases we've seen so far had at least one pure-typed starter.