Familysearch has an online index for 1871 Scottish Census… it shows there are 11,585 entries for the surname SUTHERLAND on that set of 1871 records.
https://www.familysearch.org/search/collection/results?count=20&query=%2Bsurname%3Asutherland~&collection_id=2028678 Perhaps this is that family at 152 Duke St, Glasgow…
James SUTHERLAND, 40, head (of household) married, born Edinburgh, a Printer Reader
Martha SUTHERLAND, 29, his wife, born Lochgilphead,
Elizabeth SUTHERLAND, 16, daughter, Milliner, born Edinburgh
James SUTHERLAND, 14, son, scholar born Glasgow
Isaac SUTHERLAND, 7, son, Scholar, born Glasgow
Jessie SUTHERLAND, 1, daughter, born Glasgow
Those ages are not exact matches, but …. Perhaps that is understandable if money was ‘tight’ and the shipping line was charging based on the age of the passengers to reflect the mess expenses.
https://www.nrscotland.gov.uk/files//research/census-records/street-indexes/1871/1871-glasgow.pdf According to that pdf, the enumeration book would be numbered 61, The Regn Dist would be 644-2
Here is a very long thread re a search on RChat for an Isaac SUTHERLAND, of Caithness. The Opening post mentions BUCHANAN and immigration to Australia in 1854.
https://www.rootschat.com/forum/index.php?topic=444205.0 Martin, in my opinion, the dots are simply NOT joining up, and they are so widely scattered. Glasgow is not anywhere near Caithness which was a county and it is still hundreds of miles from Glasgow, which is a city in a different county, and Edinburgh is a different city again, and is no where near either Glasgow or Caithness. So in the 1800s records there’s Caithness, and there’s Lanarkshire and there’s Midlothian to check through at least.
ADD
So from familysearch, there’s a birth showing for 27 June 1863 for an Isaac SUTHERLAND, parents as James SUTHERLAND and Mary BUCHAN… with birthplace as Glasgow.
https://familysearch.org/ark:/61903/1:1:FQ52-LFZ Scotland’s people have a birth registration, just the one in the years 1863-1865 … for Isaac SUTHERLAND
https://www.scotlandspeople.gov.uk/search-our-records What information is on that birth certificate that would show James SUTHERLAND who migrated to South Australia in 1877 as his father? .... the Dad's Occupation ... will be useful

That still does not sort the 'ENGLISH' 13 year old lad on that same voyage... but it does put some more dots on the line between William Alexander SUTHERLAND and Isaac Buchan SUTHERLAND ....
JM