hey hofosho, DWC is super easy and i think you will be happy with the end result.
anyway it depends on the plant as to weither or not to put it in the system, not the number of days old it is. generally you want to see more than just a taproot, a nice white healthy system of roots is preferable.
that said i have put newly sprouted seedlings into a DWC system, im talking just sprouted out of the peat pellet. the key is to keep the water level just below the net pot so the seedling doesnt get soaked (small seedlings cannot handle flooding like an established plant and will more often than not fall prey to "damping off") just let the airstone bubbles splash water onto the netpot and the resulting moisture will grow it out well.
as for nutes. a small plant needs very little nutrients, about 200 ppm is the standard though i personally use a little less. some people start there plants with flowering nutes because they help grow large root systems, but in my experience that isnt necassary, i wouldnt transplant anything that doesnt have roots to uptake nutes, so feeding to grow roots doesnt make much sense to me in this situation. personally i give them light vegging nutes as soon as there in the DWC kit. watch your plant, when it starts really growing (shooting upward vigorous growth, with branching) then you can increase your nutes slightly and gradually until you find your plants are happy and green with nutrients uptake in sync with vertical growth.
as for your last question, its tricky. ideally with a 600wt metal halide you want it about 18-24 inches away from the tops of plants. or as close to the tops without burning, a simple way to test is to put you hand at the top of your plants, if its uncomfortably hot raise the lights until its not, then tie a string to the reflector that length so you can easily monitor how far away your light is from your plants.
however seedlings especially will stretch toward light, and having to keep your light that far away will cause slightly lanky plants, your best bet is to put them under a CFL or T5 while there young and transfer them to your MH after they seem established and are definately vegging, this way you can put the CFL or T5 light almost right on the plant without burning it, this will keep your internodal spacing short and ultimately give you tighter more compact buds.
all that said dont follow the books, or seed companies timelines exactly they are more like guidelines, sometimes it will take longer (especially predicted flowering times) and sometimes shorter, the plant in this picture is only 14 old, from the day it sprouted from seed, and it is ready to flower, btw it was grown in DWC from the moment it sprouted
