Veg them under a 125w blue Enviro - very cheap to operate (little more than a 60w incandescent lightbulb) and you should get some nice tight internode spacing. A 600w HPS is overkill for vegging specially in the early stages and will cost you almost 10x the amount to run as the Envirolite and probably won't do as good a job! Keep the Enviro about a couple of feet over the new seedlings and gradually start lowering it down as they get bigger.
HPS systems are great for flowering as they output lots of light in the right spectrum which flowering plants need but depending on which bulb you have (some are better than others) they can cause unnecessary elongating of the stems and wider internode spacing (length between branches) because they simply don't output enough light in the blue spectrum required for vegetative growth.
As I said, it does depend on which HPS bulb you use (some have enhanced blue spectrum, many don't) but generally many newbies here simply don't have a clue that a standard HPS bulb simply isn't good enough for correct vegetative growth.
I've seen it a lot on this site - people vegging with HPS systems and the plants just get tall with wide internodes, rather than smaller with tight internodes and lateral growth and tall plants can get really unmanageable in flowering because so little of the light can penetrate lower foliage and get to the buds. The further light has to travel the more rapidly its intensity dimishes and plants in flowering need lots of intense light. As an example a 600w HPS outputting 90,000 lumens at 1 foot, outputs 22,500 lumens at 2 foot, 10,000 lumens at 3 foot and only 5,625 at 4 foot! Bearing in mind you have to use a 600w HPS about 2 feet from the plant tops, it's really not hard to see that a plant much over 4 foot tall will only be getting about 2,500 of those 90,000 lumens at the bottom and much over 4 foot will get very little light at the bottom areas of the plant.