a 400 watt light is probably too much for it, it could give off too much heat but it might not with your fans so my suggestion is if its not too much trouble install the light with no plants,turn it on for 1 day 18 or 12 whatever bulb you have or are gonna use and monitor the temps for the day...
you might be confused by the plants that look a lot bigger then yours that are around the same age its probably because the other plants are clones not grown from seed http://www.marijuanapassion.com/Gallery/showphoto.php/photo/6292/ppuser/13598 the smallest plants in this pic are 2 week old...
finally got buds http://www.marijuanapassion.com/Gallery/showphoto.php/photo/7136/ppuser/13598 some pics in the dark http://www.marijuanapassion.com/Gallery/showphoto.php/photo/7137/ppuser/13598 http://www.marijuanapassion.com/Gallery/showphoto.php/photo/7138/ppuser/13598 pics arnt the best...
just put the pics in the gallery, theres 13 Northern Lights and 9 Purple Kush clones all pics are of the same plants over the past month or so, the camera is a $20 piece of junk so quality isnt the best