All the Matches on Geni are from My Heritage, and not available to me to view the records.
On Family Search, there are similar matches, using source materials that appear to mix families:
Alexander Thomson, Vital • Scotland Marriages, 1561-1910
Name Alexander Thomson (is also linked to Jean Thomson born 1830)
Sex Male
Spouse's Name Mary Clark
Spouse's Sex Female
Marriage Date 7 Apr 1826
Marriage Place Stevenston, Ayrshire, Scotland, United Kingdom
Marriage Place (Original) Stevenston, Ayr, Scotland
Event Type Marriage
and also
Alexander Thomson
Sex Male
Spouse's Name Mary Clark
Spouse's Sex Female
Marriage Date 12 Nov 1830
Marriage Place Kirkoswald, Ayrshire, Scotland, United Kingdom
Marriage Place (Original) Kirkoswald, Ayr, Scotland
Event Type Marriage.
There are many entries with what looks like children mixed across these 2 families (NSW family and Scotland families), complete with sources.
Alexander 1861:
Alexr Thomson
Census • Scotland Census, 1861
Name Alexr Thomson
Sex Male
Age 53
Event Date 1861
Event Place Kirkoswald, Ayrshire, Scotland
Event Type Census
Enumeration District 7
Registration District Kirkoswald
Age 53
Occupation LABOURER
Birth Year (Estimated) 1808
Birthplace Ayrshire
Relationship to Head of Household HEAD
Schedule Type 29
Registration Number RG09
Piece/Folio 0
Affiliate Publication Number 2
Piece 601
Mary 1861
Mary Clark
Census • Scotland Census, 1861
Name Mary Clark
Sex Female
Age 52
Event Date 1861
Event Place Kirkoswald, Ayrshire, Scotland
Event Type Census
Enumeration District 7
Registration District Kirkoswald
Age 52
Occupation LABOURERS WIFE
Birth Year (Estimated) 1809
Birthplace Ayrshire
Relationship to Head of Household WIFE
Schedule Type 29
Registration Number RG09
Piece/Folio 0
Affiliate Publication Number 2
Piece 601
Alexander (and Mary same) 1871
Alexr Thompson
Census • Scotland Census, 1871
Name Alexr Thompson
Sex Male
Age 64
Event Date 1871
Event Place Kirkoswald, Ayrshire, Scotland
Event Type Census
Enumeration District 1
Registration District Kirkoswald
Occupation ROAD MAN
Birth Year (Estimated) 1807
Birthplace Ayrshire
Relationship to Head of Household HEAD
Schedule Type 26
Registration Number RG10
Piece/Folio 1
Piece/Folio 1
Affiliate Publication Number 2
Piece 601
Most of the matches on FamilySearch have death for Alexander Thomson as 1881 in NSW Australia, with no sources for that, but he is the same person in the 1861 and 1871 Scotland Census.
We know that 'our' Alexander died NSW 1866.
Also, Mary Clark has a death of 1877 in NSW, and she is the same person in the 1861 and 1871 Scotland Census. We know that Mary died NSW 1880.
They have 20 children listed, born from 1828 (Charles) to 1849 (Ann), including those from the NSW family. Obviously any after 1837 born in Scotland (including the last one Ann) are not of the NSW family, however Ann and others are included in the Matches from My Heritage.
There does seem to be a question (to me) about which Alexander Thomson and Mary Clark are being used to be the source for any of the NSW entries, which is why I asked.
Thanks very much,
Ros