Interesting stuff. My period of expertise is very much, much later -- thirteen centuries, approximately -- so things had changed a bit.
It makes a lot of sense that shorter bits like Paul's letters would have circulated independently and only made their way into a more comprehensive biblical composite when someone actually sat down and thought about what belonged and what didn't. The kind of arguments that i frequently make point out that this hypothetical compiler had as much, if not more, influence on the Book we read today than did any first or second century religious figure. I have colleagues doing very interesting work on biblical apocryphal, especially on those spurious books that circulated for many years with more canonical bits but didn't make it in KJV or the Vulgate or whatever.
Deacdixieboy should chime in here because his research is super relevant to this conversation.