Heya Tiwest

You can keep a 400w about 30 - 45cm above the top of the plants.

24 hour light isnt a great idea, get a timer and put it on 18/6, Heres why:
Grow Marijuana FAQ, Cannabis cultivation - marijuana growing tips & photos

Try separate those seedlings somehow, the bigger they get the harder it'll become to get them apart.

I gave my plants 1/4 strength nutes when they were 3 weeks old, then half strength after a couple feedings, the full strength after a couple more. If you see signs of nute burn (downcurling dying leaf tips) just flush the pot with 3x times as much water than soil.

Otherwise it looks good, HPS is better suited for flowering though, you want a Metal Halide for veg. I have a hps/mh ballast, can swop the bulbs when ever i want.

Anyways hope that helps
Peace :rastasmoke:
bLanK