Ok I seen your other thread now. Ya your fine man, the bulb will just slightly under perform compared to if you were running an actual 400 in a 400 ballast. Don't put a smaller bulb in it will blow up if you accidently switch it up to 1000. And like I said even if it permanently messes the bulb up some (which i honestly can't verify even happens), that's not a big deal either. Bulbs are $20-25. Like some have said you only need the one bulb right now ( or hopefully get some t-5s or even screw in LED bulbs) on to save some energy. However with starting seeds in 5 gallon pots you can't exactly fit them all under one light as i'm sure you know. I'm guessing you've had transplant issues before and want to make sure that doesn't happen? If your not careful with the way you barely water the seedlings now, you'll have all new problems in those big pots. If you over water they'll never have a chance to dry and encourage root growth, the plants will probably rot. But if you think using the other light too and spreading them out will give you a piece of mind, and you can carefully water them without saturating all 5 gallons of soil so it can still dry out, than go for it.
You could always try those smaller coco based pots first, and then set them in the 5 gallon later and the roots will make there way right through the pot. into the new soil, without doing any harm during the transplant. Good luck man!.