4 or 5 plants under 600 w hps?

Guys im in a big debate, i already have 2 plants under a 300w cfl, now i ordered a 600w hps with Digital Ballast 4D with a regulator 250, 400, 600 and super lumen!
its coming next week with the seeds, so im wondering should i sprout 3 or 2 more...
its to put in cosideration that one of the 2 plants under the cfl was a outdoor my friend gave me as it was in bad conditions, he said it was nearly a month old but my week old looks bigger lol, but it has many leaves and finally turned nice and green!
PLEASE give me advice, i have a rough idea about watts per gram, so if i would get 1/2 watt per gram i should come out with around 300 grams... but seing that im using good soil and nutes, i am hopefully excpecting slightly more... give me your opinion on what to do!
AND
when the new seeds come and i germinate them... can i put them straight under the hps? as my plants will have nearly 2 weeks of age difference.
ah and just to be said... the plant my friend gave me was attacked by a bird hahahaah so it doesnt have a main head!! but growing split up! its a funny looking plant, but looks promising.
please let me know what i should do xxD
 
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