LOL that looks like a DVD I shared with Trichome creator. Although it doesn't look like he's too popular right about now. LOL

You can flower any time you want. A bigger plant in veg will produce bigger buds in flower as long as you can light the plant entirely and have the pot size.
Seedlings will show sex if you switch em. SOG is possible because you can flower clones right after roots develope.

Long story short, expect plants to grow two or three times as much after you switch to 12/12. Plan on this to determine when to flower. Keep in mind how much light is available because light intensity deminishes dramatically the further from the source you get. There's no sense in growing a six foot plant if you can only light the top foot and a half of it.

Laterz