Robpiat -
I'm not qualified to answer the growth rate question. I'm not sure if there is a standardized growth rate, or if smaller mantis grow faster than larger mantis, or if males get larger than females, etc. I only have experience with two O. scyllarus mantis, a male and female. The longest I have kept a single specimen was 9 months. It was 6" when I got it, and about 6" when it died.
Jimbo -
I've only seen four O. scyllarus for sale, and all were 6" or more in length. Around here the prevailing attitude seems to be that they are pests, and if someone wants to buy one, they are more likely to want a large pest than a small one.
The camo colorations sounds cool. Got any pics?