great to hear from you Medusa!
I'm in Brisbane.
John Small's parents seem to be David Small and Margaret Sutherland and his grandparents seem to be John Small and Jannet Tait.
Why do you think he came on the City of Brisbane? It would make such sense if it was true. There are records on microfilm and cards at the Queensland State Archives in their shipping section about the City of Brisbane, including crew who signed on at Plymouth. John Small wasn't on that list, but it does look as if there was a fold in the original where you can't see what was written. Maybe he got on somewhere else. There's also information there about an enquiry about the City of Brisbane, but Thomas Markwell's testimony seems to be missing. I've now learned there's an inquest there into Thomas's death, but I haven't looked at that yet.
Bill Barlow wrote a book about the voyage of the City of Brisbane, in more than 1 edition, that's in the Brisbane City Council Library and the Queensland State Library. I should check it again as it has complete passenger lists, with, in the first edition, Thomas, "Hannah" (Harriet?) and Mary transcribed as Harkwell and the rest of the family as Markwell. This was corrected in the next edition. A lot of the shipping records from that time were damaged in the big 1893 Brisbane flood.
Please let me know if I can help more. See other threads I've posted in for more information about Thomas and Harriet. Thomas's headstone from Paddington or Milton cemetery seems to have been buried in a gully at Toowong Cemetery for fill!