I agree that this isn't 100% ROS related. But we run a few GoPro's hooked up to ROS in our lab and I think it's useful to have the solution to this on here.
None of the GoPro's we use can stream video over USB, they can all do it over HDMI though. We use these Magewell capture devices to capture the HDMI stream and present it as a USB webcam to the computer. They're linux compatible and simple to use, the down side is the price tag, especially if you're going to use 6 of them!
The next challenge you're going to have is the data rate of 6 x 1080p captures. This is probably beyond your average PC at the moment and definitely beyond a beaglebone black. If a reduction in resolution or frame rate is acceptable, then you may be able to make it work.
Regarding Wi-Fi, we tried it on a single GoPro and the resolution and latency was so bad that we didn't consider it.
Can you rephrase this as a ROS-related question? There's over 40,000 questions on the site and with so many we aim to keep them ROS-related. Otherwise, you may want to ask on a more general site such as https://robotics.stackexchange.com