There is no need for an intake fan, as long as there is a way for air to get into the growspace it will come in as the stale air is extracted. It really is amazing just how many people don't realize this simple fact.
If you have a 500cfm exhaust, then putting an 80cfm fan as an intake is going to restrict the intake, as if it was allowed to flow freely, it would be moving 400cfm more!
Temperature control will be much easier if the cooltube is run as a separate, sealed system ie: it has it's own intake, ducting, fan and exhaust, and the air running through the cooltube never actually gets into the growroom. I'm not sure 80cfm is enough to cool a kw light, it would depend on the ambient temperatures, and the type of fan (if it's an axial, it almost certainly won't be enough since they can't deal with static pressure like sucking/blowing through ductwork)
That said, the fan you have is more than enough to keep that size room in acceptable range, and could well be used to cool the bulb. I would try a 'dry run' without any plants in there, try a couple of setups and monitor the temps (ambient as well as in the room, ideally you should be able to keep it less than 5 degrees above the outside temp)
I'd start by running the big fan pulling air across the bulb through the cooltube, and blowing it out of the room, leave it shut up for a few hours, then come back and see how warm it is. It should be ok, to my mind, but will depend on the temperature of your intake.
Sorry for the long post, hope it helped.