Earliest time to start flowering?

Hey all. I'm considering a PC case grow with 1 to 2 plants. Roughly 100 to 150 watts of cfl's, changing bulbs for appropriate plant state. My main question is this: I know all strains are different and some are meant to be flowered at a small hight, but generally speaking, when is the absolute earliest you can switch from veg to flower? What would happen if you initiated flowering when the plant was topped once and was about 8 to 10 inches high? What about even 6 inches? Also, could you give budding nutrients to such a small plant? Thanks!

you can flower at 6 inches! it will grow 2x or even 3x that size!
my auto flowering plant started flowering at 3 inches

If you have a bagseed or some other unknown strain you'll probably want to veg it out for at least 4-5 weeks unless you want to risk a hermie and have a ton of seeds.

With a PC case, I would top the 4th node, and then FIM the two new branches. Keep it going for a month and when you switch to 12/12 it may double or triple in height. After 2 weeks of 12/12 you can top one more time to control height. It probably won't grow any taller after this, but by doing this you just added an extra 1-2 weeks to the flowering phase.