I would wait a few days (maybe a week) before putting new sprouts under a HPS light.
They really dont need that much light to start with, and should be ok with natural light untill they are a little stronger. The HID lighting will promote vertical (stringy) growth but I like bushy plants with horizontal growth so I like to ween the plants onto HID lighting gradually. This is just my pref option, if YOU are able to manage the heat buildup then theoretically you could give the plants 24hr HID lighting in the first week but IMHO this is overkill.
maybe consider investing in a cheap fluorescent or CFL light. No heat and promote bushy growth for the first few weeks. I would then hit it with the 400wat HPS half way through veg and then for flowering.