nice setup in that growtent mate, with a 600w light in there your do well.
and i wouldnt do to many plants in there, what is it the 1.35m size?
and are you planning to use that MH lamp for your flowering?
MH lamps are the doggs nuts for the veg/grow stage.
but for flowering id use a HPS lamp, still in your same coolshade(just change the lamp/bulb) to a HPS the same watt, for you a 600 for flowering.
lots of people are useing dual spectrum HPS lamps as these have both
blue and red spectrum of light. HPS lamp like the
SYLVANIA GROLUX LAMP are good for both veg and flower so no need to change the lamp/bulb.
metal halide lamps, blue spectrum( vegetative growth)
high pressure sodium lamps, red/orange spectrum( flowering
you can use you MH lamp for flowering but if you can get a HPS lamp id get 1
anyway hope this helps

smokedogg