my room its about the same size i use a 400w lumitek mh/hps ballest.Sometimes i have trouble w/temp depending on temp out side.600w would be better it u have a good way to control heat.
I use canna terra its the best nutes ive evr used my plants seem to love it got a mangokush,spaceshipand nortehn lights 5 weeks in flower use canna flores,cannazyeme,cannaboostand pk 13/14. great system will post pics later
I wouldnt worry about the yellowing leaves thats typical for this time in flower being as the plant useing all its energy to form nug.
And as to the heights i would find somthing to put the shorter plants on to raise them up.
I use 3 25 cfm fans 1 intake freash outside air and two exhust. i got them from buyextras.comBXT Quiet 80 x 80 x 25mm (3.15 x 3.15 x 0.98 inch) Fan + AC Power Supply 179699781 2 $13.95