Veg them out much longer than you normally would, or take Daihashi's advice and think about filling the room with more plants to occupy the empty space. Much better returns for the space and amount of light you want to use. I also agree that a 5x5 room would do well with a 1k HID. I'd go with a cooltube for both the 1k and the 600w, no mater which you decide to use. Ventilation is very important when working with HIDs, so try to spend equally as much money on it as you would on your light.