Yes Rod Stewart did sing with John. After the demise of the Hoochie Coochie Men (the old Cyril Davies Allstars) John's manager and agent at the time George Webb, he of Dixielanders fame put together a group called The Steampacket. This comprised of Long John Baldry, Julie Driscoll (now Tippet) and Rod Stewart as front singers backed by Brian Auger's Trinity. All three singers of course went on to top the charts and although I don't have much info on Brian Auger I believe that he went on to have a lot of success.
I believe that John's first group was the Thameside Four, it was in that guise that I first saw him, and he later became a regular at the Ballads and Blues Club in London. He also sang with the Ken Sims band during the "Trad" jazz era and was offered a job with Acker Bilk's Band when Acker was riding high. He came back from Germany to take up the offer but for some reason decided against it at the last minute.

Although George Webb himself was/is a died in the wool revivalist jazzman and still playing he certainly should be given credit for spotting that group of musicians, seeing their potential in a field outside his own and putting them all together under the Steampacket label.

As for John road managing Gary Davis, that raised a smile. With all due respect and not to speak ill of the recently departed, at that time John could barely road manage himself. R.I.P John.