I would plan on 1.5 weeks for the seedling stage. Then 6 to 7 weeks veg at 24/0 or 18/6, I use 24/0 for veg.

Get the Fox Farm family of nutes, follow the watering schedule closely that comes with the granular set of nutes. Do not feed your seedlings anything for 1.5 weeks.

With a 600w MH keep it 20'' or more from the tops of the plants not sure with air cooled though.

Kimit is right 2-600 watt HPS's in an 18 sq ft area is alot, I do see your situation with length though, maybe consider cutting it in half with a black out curtain, and have a veg and flower area. Just a sugestion on this.

Take cuttings of every plant and cloan before you flower because you may have a jem of a plant and won't know untill 8 or more weeks later and cloaning then is very difficult and a much longer time to get them to the veg stage. You can always toss the ones that are male and don't make the cut.

BOL on your grow and keep posting with your progress.