Tricky to water those little plants in those big pots. You may not have to water for a long time and when you do just touch up the surface as needed, while making sure the bottom doesn't dry out too, lol... Test a pot full of dry or slightly moist soil so you know what it should feel like when you lift it and judge the moisture level by weight. The soil should just be evenly moist and you should be able to feel a little bit of moisture if you stick your finger down an inch and a half or so -don't overwater!!!!!
There's no real answer set in stone about when to put it under the 600 W light. Maybe when it's 4 to 6 inches tall? Put it a good distance away and give it a little time to adjust.
Edit- do you mean you are using a HPS as your veg light? Or are you asking when to put it under the HPS for flowering?. I thought at the top you were saying you have a 600 W metal halide for veg. Either light is ok for vegging really just not sure about your question
You can probably live without the intake on in there for a little while as long as the plant's getting some regular source of fresh air, and an oscillating fan is good set on low, and fairly far way for now - keeps the stem good and sturdy