Denfinetly put the 400hps. Alls you will have to do is use it for about 2-3weeks more and then flower. Hydro is fast so however big you want it shouldn't take long. But remeber hps make plants grow lanky and tall. So you might want them to be bigger then usual to get the same yeild if veging with it. The hps is 100 times better then 120 watts of fluros. I would use both tho. Like they said, make sure you keep the light far away when they are first introduced to the hps. At least 3 feet away so they don't get hurt. They are only 2.5 weeks old. Then slowly move it closer every few days. 1-2 day should be good, before you know they will be almost touching it because they will be growing fast. The light should never be closer the 15-18 inches when they are are happy with the hps and growing.