[MarsHottentot]

I think it won't be quite as hard as we might believe as some characters may just play a support role. Silver (the "emotionless warrior") seems to be set playing the straight foil to Gold's comedic antics, while Black is a robot wrestler who might just be an automaton - note that Gold and Pink are listed as "androids" or "cyborgs" (I can't remember exactly) but not robots. I think that, at this late date, the term "robot" is morphing from a catch-all phrase to a specifically non-sentient, worker / tool term. Obviously, I could be way off, but I find it significant that Black is identified as a "robot" and not an "android". He could be a weapon on two legs, essentially, with little need of a deeper back story or further development.

So, that being said, your focus potentially narrows down to 7 - still a LOT of characters to develop, mind you, but less daunting than 9!