Yes, you need an inline fan for your cool tube. I think that for the price, a six inch fan is going to be a popular option. I tend to like the HTG fans myself since they seem to be very durable and they are somewhat decently priced. HTG sells one of their inline fans as a combo with a fan speed controller that way you can tone it down just a bit to keep it somewhat quiet. http://www.ebay.com/itm/6-INLINE-DUCT-FAN-SPEED-CONTROL-COMBO-Exhaust-Blower-Controller-speedster-can-/120861760398?pt=LH_DefaultDomain_0&hash=item1c23ec1b8e
Also, if I did 300w of MH would that be similar in heat to 325w florescent? Would I absolutely have to vent from the closet or would there be another way? Just until I get the tent set up in its location in about 2 weeks. I don't want to have them go the rest of their veg state with the florescents but I'm still iffy on running HID in the closet.
I always bought my ducting at Menards which should be the same as Home Depot, Lowes, and Home Depot. Six inch ducting would be in the heating area of the store and should come in lengths of 25 feet. Then you just connect the ducting and cut any extra off.
I can't really answer on the heat since I don't know with forescents, but that 300 watts of MH would be more light than 325 watts florescent. You would have to have that ducting running out of the closet so that it can dump all of the hot air outside of it. Otherwise you will just create a sweat box and it will get very hot inside of there. I ran a 400 watt HID in my closet, but I had six inch ducting running out of it so that it would deposit the hot air outside of the closet. Then that would start to warm up the room a little which which isn't much for decent ac to take care of. Or if its winter it don't matter, in that case the heat will just keep you warm.