there really is no rule of thumb to vegging, you can go as long as you'd like, or as short as you want. I DONT throw plants in flower that are under 9", and most would agree to let them get even bigger before hand. I like to throw little bushes into flower
something along those lines is what I like to veg too...those HAVE been topped, in fact they've had a couple branches cut on each for clones.
I dont understand what you mean about feeding, for your application in soil packed with nutes already, you would feed plain water for a week or two, maybe not two weeks, but close, then start alternating feed>plain water>plain water>feed. Feed being your nutrient mixture.
If you just continually feed nutrients you will burn the ever living hell outa your plants....and we dont want that.
Most people feed based on PPM or parts per million, or commonly called EC (electrical conductivity) or TDS (total dissolved solids)...all the same thing really.
your common tap water comes out at 200ppm or round abouts. When you start to feed you want to gradually work the does up acclimating you plants to the new feeding schedule.
and yes 12/12 with the hps when you toss them into flower....make sure the room they are in is totally dark when the lights go out for the other 12 hours or else you will be dealing with hermaphrodites....do you plan on cloning these strains to keep the genetics?
on a side note....yay! for me 200 posts!