One fan can cool 3 of your HDD's. Those seagates have 2 usb ports on the front panel. You can dial your fan there.
This is my mini rig:

But for some reason something went wrong. My ping went too high. I started to have connection interruptions, started to get too many rejected shares and after a while wifi just didn't work at all. I couldn't keep it going. Whether it was a windows 10 problem or my usb wifi couldn't handle the statical electricity which was generated by the HDD's and my GPU I don't know. The problem was one of those I am sure.
Hi, your setup looks very cool as well. Could you please share me what's that monitor? does it have HDMI? And you mean the rejected shares is for GPU right?
I am thinking to rent some of my HDDs to Storj but as far as I know it may take very long time to fill all the storage with Storj.