I see that Marion/Mirren was a widow in 1851, and that Angus was aged 9, which means he was born 1842/1843, in Paisley. His youngest sister Marion was aged 1 in 1851, so she was born 1849/1850. Therefore Charles must have died about the same time.
The 1841 census says that Charles was not born in Renfrewshire.
His age was given as 40 in the 1841 census. In that census, adults' ages were supposed to be rounded down to the nearest 5 years, so he could have been anything from 40 to 44. If his age is accurate, this means that he was born between 8 June 1796 and 7 June 1801.
Noting that none of his sons was named Neil, and none of his daughters Grizel, I think you can probably discount the Charles Galbraith baptised in Campbeltown.
This does not necessarily mean that the one in New Kilpatrick is the right one, because there are many people whose baptism records, if they ever existed, have not survived.
His eldest son was John and his second daughter was Mary, so there is a chance that his parents were John and Mary.
In the 1841 census there is a Mary McMillan, aged 70, in his household. Could this be his mother, perhaps? There's a marriage of Donald Galbreath to Mary McMillan in Gigha and Cara in 1796. Could they have been his parents?
Marion Galbraith, other surname Graham, died in Paisley in 1888 aged 84.
Angus Galbraith was still in Paisley in the 1901 census. He died in Paisley in 1918 aged 76.
