Hi Chisholm
Here are a number of ideas for homework before heading North.
Inverness has just opened a new archive at:-
The Highland Council Archive Service
Highland Archive and Registration Centre
Bught Road
Inverness
IV3 5SS
I believe this covers Ross and Cromarty.
Caithness is served by the North Highland Archive at Wick.
http://www.highland.gov.uk/NR/rdonlyres/0F21430E-C020-44BC-9966-09C5FAF80622/0/nhaleaflet.pdf The staff and volunteers are exceptional and are happy to talk over the phone on what help they can offer. If you were coming in the summer I would advise booking as the search room is very small.
They hold census and parish regs on film amongst other items.
Another good site is
www.Caithness.org for all things over the Ord.
You could try
http://www.caithness.org/community/museums/nha/index.htm with family history button.
The Caithness Family History Society page is
http://www.caithnessfhs.org.uk/index.htmThey would be the best to contact before a visit as society also hold records.
CFHS have their own room in Thurso Town Hall ( now Horizons) and this should be manned Wed and Sat afternoons but you should check before arriving.
Thurso Library in Davidsons Lane holds film of the Caithness parish registers.
I do have a problem with your ROSS family though. Caithness is well served on family search web site and I cannot find this family in Caithness. I found the marriage of Andrew ROSS and Christian CHISHOLM at Dingwall in1819 and was wondering were the Thurso information came from.