I generally tend to standardize the spelling of surnames in my genealogy file, although, I always transcribe records with the spellings as they appear (or as I decipher them) in the original records. My copy of the marriage of James and Ann in 1798 shows the surname spelled McBeath rather than MacBeath (FHL #0990563, OPR 52/1, p. 85).
I saw several of your posts on the SCT-Sutherland mailing list at Rootsweb. I noted that you showed the eldest son of James and Ann as Neil, born in 1804. I hope that since those posts you have located the birth of their eldest son, John, who was born 8 May 1800 (on page 24 of OPR 52/1).
My tie to this family is through Neil McBeath and Catherine McBeath. They had two sons born in Kildonan (1797 and 1800) and at least one son born in Brora. FYI, Neil's occupation was listed as "soldier" on the death certificate of one of his sons. I know that this Neil McBeath didn't serve in the 2nd Sutherland Fencibles, so I presume that he served in the 93rd Sutherland Highlanders.