yes you can start seeds on a 12/12 schedule once theyve been germinated. i can tell you though that even 2 weeks of veg is better then none. on a 12/12 the plant becomes stunted but will still grow, slowly. ive done a 12/12 from seed before and got results, but found that another seed i had germinated and placed into veg about 2 weeks later, was bigger, and had a faster growth. also your goal of 15g dry is also attainable through CFLS as i did it myself. i dont think any other safisticated lighting system could fit in your friends cabinet. although 2 feet high is still mighty small even for CFL.