thank you for your help. The family you found are very close, aren't they!
The Catherine Scott I found (below) arrived in 1867, aged 17, and with no family.
If the names are just not recorded properly on Catherine's and William's Marriage Certificate, it could be them. I have noticed that about tracking names. The census you quote is 1851, but Catherine is not on the list.
I see a George Scott in the 1861 census in Scotland who is also a coachman, but he is a lodger, and has no family there. The age is correct: both born in 1813.
And Martha Scott is also in the 1861 census. A coachman's wife. But Geotge is not listed with her, which figures because may be that George who is down as a boarder somewhere else in the 1861 census. Also there is a 10 year old Catherine with Martha now in 1861.
There is a "Cathe" Scott aged 17 from Scotland, on the JOHN TEMPERLEY 1867 passenger list (Departed Plymouth on 2nd July 1867). The JOHN TEMPERLEY arrived in Hobsons Bay (Melbourne, Victoria). The date is about right. Trouble is, she is still 17 in her mariage certificate in 1869. That is not off by far, though. And there is no-one else coming in who from Scotland with that name in that year.
I could not find the passenger list of the JOHN TEMPERLEY departing Plymouth, only the arrival passenger list in Melbourne.