When do I switch to 12/12 with plants of different sizes?

Hi friends,
Twenty-three days ago, I transplanted 30 seedlings from Jiffy pots into 6" square pots. I'm trying Sea of Green in a 6' x 2' closet with a 400W MH bulb ( I have a 400W HIS to switch to when I go to 12/12). I used two different brands of commercial potting soil, and after a couple days I noticed the seedlings in one of the soils were yellowing and not thriving. So, on day six, I dumped the weak, yellow plants and replaced them with new seedlings from the same batch planted in fresh soil. To my surprise, the new plants also yellowed but on the advice of some wise growers here on Rollit, I gave the plants a dose of Nitrogen and, voila! the yellow faded and they began growing.
Now, the original plants were transplanted 23 days ago, the replacements 18 days ago, but they are more than 5 days behind because the nitrogen wasn't applied until 7 days later.

So phase 1 and phase 2 are both thriving, but the first plants are now about 17" and the rest are 10" (Incidentally, my prior crop, using the same strain of seeds, were much taller at this point. Is that because they were in 12" pots? Or because they were less crowded from the start?).

I do not have the space to divide the plants up, so that option is out. I'm going to have to decide on a compromise date to make the switch to flowering.
I would like to wait until I can sex them, because I plan to thin them down to 12 plants and it would be nice if they were all females (Last crop, one of my females turned hermie and before I knew it, I had 10,000 seeds and sensemilla was just a lost dream).

So, my fellow Johnnie Marijuanaseeds, how would you deal with this situation?
 

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