As far as burning them goes, you just need to make sure the light is high enough above the plants, you'll have to figure out exactly what is gonna work for you. With my 1000w, it had to be around 2.5 feet above my plants. I didn't have an inline cooling fan though.
It is best to figure on 1 sq. ft. per plant, in a 2x4x5 tent, that would provide you room for about 8 plants. The 600 may be fine for that, aslong as you can get rid of the heat. If its only 5 feet tall, your gonna wanna keep them short like yours are now. That way you don't overgrow your tent when you bloom, or end up with the plants getting to close to the lights!