Seem's like you have plenty of room in the grow room so just keep vegging till the plant's reach a height that will give you a good yeild about a ft to a ft and harf i found to produce a good yeild.If the plants are all growing ok and it looks like they are then add nothing else,other than up your nutes,but make sure that you flush so that no salt's get the chance to build up.When useing two light's like you are wanting to do like a mh for vegging and hps for flowering then i would flip them at the start of flowering,Make sure that you keep your light has close to the plant tops has possible without burning the tops.This will mean the differance between a big yeild and a far smaller one,when i first started growing i was well into topping my plants sometimes even multiple topping more than one section of the plant,but over the past few years and through been around alot of other growrooms i have noticed that not alot is gained from topping infact it can knock you back a week or so.I have done test my self on this two plant's grow under 2 1000 watt hps grown in near perfec conditions and the plant that was not topped produced a little more than the topped one.Sometimes i think topping can cause a plant to strech,but each to there own,I mean after all i take all the buds of the main stem once dried any how and cure it.Go with advanced nutes on yr next grow there are alot of haters of advanced on here and i was put of for a number of years due to this fact,but since flipping to advanced the yeild is well up to 25% bigger and the plants love it.Advanced is ideal for the newbie because it is a matter of adding the right ammount to water and the ph will go to the right one for the stage of growth and the ammount added,also there is no need for a ppm meter has the same happens with that has long has you know the ppms of yr water.good luck get the light lower its a tad high and start to give them good ammounts of nutes and you should have a bumper yeild,i would also pull them tighter togeather till they get bigger so they are getting more light good luck any questions post me tyke.............................................................