Well first off, good luck.

It is possible to grow a decent plant around 12" tall using 12/12 from start to finish.
I did it with white widow and yielded an ounce.

You will want to make sure you can get your plant right up to the lights as I also have had a plant in 12/12 from seed that wound up over 2' tall. I did not have the lighting close enough and it stretched.

If you veg first, how will you know if they are female? My guess is that by the time you get pre-flowers you will have used up your 12".

I would go with 12/12 right off the bat and LST. if you are lucky and get preflowers early enough you may be able to wrap 2 girls in that space but that would be pushing it. also be wary of heat issues in a small space, evan with floros.