Which arduino/s should I get as the main hub?
The Leonardo and the Micro both can act as a USB MIDI device.
Is it possible to attach a 5 pin midi connector as output instead of usb?
Lots of projects on the net including mine http://www.thebox.myzen.co.uk/Hardware/MIDI_Shield.html
and how would I chain 7 together, I'm looking at this image:
Repeat that circuit but instead of sending the input to A0, send it to A1 to A5. I know this is "only" 6, which will give you 96 pots. To get an extra one connect another chip's output to one of the inputs of a multiplexer.