I don't want to be an ass but...
"too many CFMs makes for inefficient cooling"
CFM= Cubic Feet per Minute . The more air you pull out from a hot room , the cooler it will be....
"" You should in theory buy an 8 inch fan to go with it (buying a smaller size fan with an adaptor will wear out...
After looking at your picts once again, here's what i would advise you do:
Currently your fresh air is pulled through your hps, then into that vacuum (the vacuum alone creates a slightly warm exhaust btw (air is run over the electric motor to cool it), then dumps into the room.
Try this...
Looks like we have similar rooms (I have twin 2x3x3 rooms) and set-ups, and heat issues. I fixed mine so let me tell you what I see wrong with yours:
I see your HPS ballast is in your grow room. Very bad. It produces major heat, plus, if it can't cool itself it will start a fire. Best to...