Oh, and btw this is the soil I am now using.
And, I just realized I can raise my entire box with bricks to avoid having to LST my plants. I initially took the bottom out to have more heighth inside and didn't realize this design bonus until now. I took a trash bag and duct taped it to the inside of the box, which makes it waterproof to the floor and lightproof at all bottom corners. I also just acquired a BBQ cover from my friend to use when the lights go out. It's a perfect size, and will take care of all light leak issues for flowering.
As far as when to flower...I would flower when they get approx half the actual growing height you have in your cab. Example...if you have 40" I would flower when the plants reached 16-18 inches. Since you are using CFL's you can get those lights real close to the plants.
I can't really tell but by the pics but it appears to be more of a N or Cal-Mag issue. More than likely a N issue which your grow big should take care of. Also, the medium you are using is good stuff based on the pic of what's in it. That should also help you combat any lil defeciencies that show up. As far as when to add more soil...you can just add it to what is already there and roots should form where your leaves are. I personally wouldn't do that at this point. Just get'em big enough to take clones and you can plant the next run with full pots.
Remember, your main goal should be to learn how to get the plants to harvest. This is your first go so keep your expectations low and lets get these girls to harvest. At that point, your experience and skills will be much better and you can learn from your mistakes for the next run. We just gotta get these girls to harvest which I believe should be no problem. Don't worry about yield or any issues that crop up. You are in soil and the "fixes" are much easier to apply in soil/soiless mixes.
Give'em the grow big at half dose and lets see if that yellow stops. The leaves that are affected more than likely won't get any better so don't fret over that. She will grow more leaves.
I certainly thank you for the offer of smoking me out but I get my high off possibly helping a fellow grower out. It sends nothing but good Karma and we all need good Karma when dealing with the trial and tribulations of growing this fantastic plant. Lets get these girls to harvest...that will be a big enough high for me.
It soumds like you are diligent enough and have the necessary skills to improvise when needed. Thats a huge plus in your behalf so just keep up the hard work and lets see where this goes. Let me know if I can help out as I will be checking on your grow. It's fun and exciting and I certainly remember my first attempt. Those are good memories. Good luck and lets see how they do with the first taste of nutes.