The 12 shinshou seem to have originated in the theory of the five elements as it was imported from Ancient China. According to a couple of sites, this theory was lost over the ages in China. The original 12 shinshou started as names of various gods that represented different mansions/constellations used in divination in China and these were then brought to Japan and passed on through various divination methods including rikujin divination. The original depictions of the 12shinshou in China were of animals, beautiful women or great noblemen, but this slowly changed over the ages in Japan as the shinshou changed from being heavenly beings to being the kishin (fierce gods) that became shikigami. (It should be noted that written records of the shinshou being Seimei's shikigami arose after Seimei's death and were not from records of the period).
Interestingly, sites mention that Kijin is a *man* compared to the female characterisation we see in Shoumyou. ^^;
Some century, I'll actually get around to compiling everything and putting it up on the website. It's planned for, but time is elusive. >.