well, I've got nothing but respect for Nick, but I still think any adult carpet needs more than 12 deep. Indeed the males tend to be much smaller, but I still think even the "smaller" ssp, (jungle, ij), need more than that. post a pic of your carpet, maybe we can figure out the ssp.
yes, see how it goes and upgrade as necessary. I never put any snake in it's adult sized final cage until it's pretty much fully grown. coastals probably go through 6-8 enclosures before they have their final home. I grow the cage with the snake no matter what species.