I understand the method in Scotland is to use parish number, ED and page and I assume the 1851 reference for James on the ScotlandsPeople website "346/6/28" is in that format.
Yes and yes.
Since the source of the Scottish census records is the General Register Office for Scotland, I don't think it is necessary to try to match the referencing system to that adopted in England and Wales, and there is no standard method that I am aware of. I think prefixing Scottish census with RG would be counter-productive because the 'RG' stands for 'Registrar General' whose remit did not extend to Scotland.
I would suggest using
GROS/yyyy/district number/ED/page though if you would find it easier to use
SCT/yyyy/district number/ED/page there's no good reason why you should not.
For clarification, I have said 'district number' rather than 'parish number' because the boundaries of districts in urban areas don't always conform to the boundaries of parishes, and in some cases (e.g. Dundee) a parish can be divided into two or more districts for purposes of registration and census. Also the boundaries of census districts, and of EDs, do change over time.
PS I take it that you have viewed the original census at Scotland's People, not just relied on the (possibly mis)transcribed versions available online?
PPS I see that Jessie's daughter Isabella was born in Glasgow on 13 August 1880. She is registered as Bella Calthorpe, daughter of George Calthorpe, gunner in Royal Artillery and Jessie Calthorpe, maiden surname
Campbell, who were married in Glasgow on 4 February 1880. However the marriage indexes her as Jessie
Kennedy. If you take a look at the marriage certificate (Camlachie 1880/644.2/61), available to view at modest cost at
www.scotlandspeople.gov.uk, it will tell you the names of Jessie's parents, including her mother's maiden name.
Given the ambiguity of surname and the fact that she seems to have been brought up by her grandfather, it would not surprise me to learn that she was illegitimate. If so, there could be a record in the Kirk Session minutes of the parish of Dull. These are available to view in the Historical Search Room in General Register House, Edinburgh and in various local archives. They are supposed to become available on Scotland's People but I do not know when this is likely to happen. In the meantime, if you cannot get to Edinburgh you could hire a searcher to find and transcribe the record, if it exists.
You can look for James Campbell in the 1841 and 1851 censuses at
http://www.freecen.org.uk/cgi/search.pl