FWIW - I kept a pair of WC reidi for almost 5 years, (a flourescent orange female and a purple male.) They measured 11" and 12" respectively. I realize WC fish are usually larger - none of their babies ever reached their size either, but I certainly wouldn't have condemned them to living in a 12 gallon nano.
Also, you're not comparing apples to apples. You, with your several years of reef and seahorse experience can push the envelope a little, and happily keep 2 pair of horses in a 26 gallon tank. Someone without your experience is only headed for disaster.
I stand by my comment that reidi are some of the larger species, and that none should be kept in a 12 gallon tank.