I've run under 24/0 and 18/6 with no noticeable difference.
Regarding plant life, I've read numerous times and in numerous reliable places plants being kept alive for many, many years.
Also, the growing three times the size thing? We typically refer to that as the "stretch" period. Most strains will stretch 2-3 times their size when flipped to 12/12 within the first 21-24 days.
-spek