As I stated previously Kharn has been 'designed' as more of an evil or anti-hero fictional character.
He started out his life as a typical human but at some point in his early years favored the Stomatopods and their home, the ocean.
Whilst what you see rite now of Kharn resembles something akin to a 'cowboy', the clothing is in place to hide obvious things, he was designed in part like 'Davy Jones' (Pirates of the Caribbean), which may sound confusing due to the lack of "ocean accessories". In short Kharn can manifest himself or part of himself at will into something more Stomatopod related...
With this in mind Kharn cannot change into a Stomatopod itself, it is more of a mix between the two species of **** sapiens and Stomatopods, thus even in his changed "form" Kharn will still have an overall "humanoid" shape...
The plan for his upbringing is going to have a mix of my own experience with keeping these animals from when I first got them to where I am now and some additions, along with fictional conflict which actually leads Kharn to the decision of abandoning his family, friends and mankind itself.
As for the getting to know your characters better, there are plans to utilize another idea from a film in place today, 'Rise of the Planet of the Apes' has a broad list of 'ape actors' who play vital roles in the film which allow the audience to 'feel' more about not only them but the lead ape himself 'Cesar' played by the master of animation Andy Serkis. The same or similar thing would be in effect here, think "leader for each species" and their own 'captains' etc. This can allow the same effect, where a more emotional detail can be attained through individual stomatopods rather then them all like some 'terminator' killing everything in sight...their being led by something that knows mankind very well.