Most gonodactylids that might be in live rock settle at 7-9 mm total length and have good color and are sexually differentiated at 11 mm. At this size, they most every 2-4 weeks and grow about 10-15% per molt. I have some pictures on my computer at work and will try to post some tomorrow, but there really isn't much difference in appearance. A juvenile stomatopod once it develops color (less than a week after settling)is just a miniature adult.
Roy