In the hot summer months your inline fan will only cool temps to a certain point. I would even use the word cool, more like less hot. I have a 6" fan with carbon filter blowing through 4 hoods all on 750w. Ive managed to keep temps in the low to mid 80s because I'm in a basement...but its getting really hot up here in new england and I have hit 90° in the grow room. So I went out and bought a window unit a/c and built up a box in the back of the a/c to enclose the back of a/c. Cut a 6" hole in box thats connected to ducting and booster fan sucking the hot air out ac box to and out of basement window. I might actually grab a 8 or 10" inline fan and reduce down to 6". I'm sure someone already said this but yes, what you want is called a reducer so you could run a 8" fan to your 6" ducting and hoods. Good luck dude, summer is tough for growing. A/C is a must, I feel. I've gotten away with no a/c for a while but realized it must be done to get optimum results . I'm actually going down to finish up the box and get my a/c cranking in grow room as we speak . Also add a booster fan blowing into your grow room to get some more air flowing in there, also add a few oscillating fans. Best things to do when temps are high is to have humidity in check and have proper air flow. Good luck and stay cool!