yeah it will weigh down the hood too much, you want to hang it off to the side and put some piping between it. if you dont need much often grow shops will sell you a length.
I hang as little as possible above my ladies. Besides even the best fans have some vibration, mounting the fan directly to the hood will allow this vibration, even if slight to vibrate the filament in your bulb, i doubt this would do it any good.