That's quite an ambitious project you've undertaken, Tim.
Lots of information there.
So much information that I'd suggest inserting some paragraph headings to break up the monolithic text and help the reader navigate. It would also be helpful to beginners to pop in a brief summation and recommendation at the end of each topic, e.g.
Part One: Connecting your external audio interface USB{stuff about USB}
Firewire{stuff about Firewire}
Thunderbolt{ditto}
Bottom line: USB 2.0 will do the job for most home recording setups, is the most widely-supported type of I/O, and usually adds no extra cost to your build.
IOW, make it convenient for the lazy reader while not glossing over the details for the serious nerd.
Great work. If you keep fleshing it out like you've been doing, this will be a valuable resource and the kind of page that gets linked to often.