Edit: I see that while I was typing this that you have come to the same conclusion as I have. But I'll let it stand anyway.
Dwelling: Little Kilmundy Bothy
Adam BALLENTINE
John TOSH
Charles SMALL
This is the only Charles Small in the 1881 census that matches with profession, age and location. So I believe this Charles Small above to be the correct one.
A reasonable assumption.
On searching all of Charles Smalls born in Scotland on SP in 1857 or 1858 I get nothing back at all.
This leads me to a few lines of thought - either he was born with a different name OR the age stated in the 1881 census is incorrect.
Also reasonable.
I look at who he's living with in 1881.
I doubt very much that this is likely to be useful. These three young men are living in a bothy. A bothy was provided by a farmer to accommodate his unmarried male workers, and sometimes married ones if their job was too far from their home to go home each night. So generally there is no reason to think that the occupants of a bothy are related to one another.
In 1883 a Charles Small marries a Harriet Patullo in Forfar. In 1881 Hariet was 18 years old and living in Forfar with her family.
I think your next step is to look at the marriage certificate of Charles and Harriet. This will tell you the names of their parents. Once you have this information you will be better placed to find him.
In 1891 Charles Small, 30 and Harriet Small, 28 were living in Inverarity with four children. In 1901 Charles, 40 and Harriet, 39 are in Lintrathen with seven children. They are still in Lintrathen in 1911 with eight children, Charles aged 49 and Harriet 47. You might also like to look at one or more of these later censuses to see where Charles said he was born.
In the 1871 census there is a 14 year old Charles Small living in Coupar Angus. He is living with his father John Small aged 57 Gardiner and his mother Margaret Small (can't make our her age). There are no other children living here, it is just the three of them. In this census it states that Charles was born in Coupar Angus not Rescobie as it states on the 1881 census. But then again I don't know if this is the same Charles
Probably not. Charles Small, son of John Small and Margaret Grant, was born in Coupar Angus on 29 April 1856.
There is an 11 year old Charles Small living in Forfar in 1871.
The place of birth is correct, but the age doesn't match up with the 1881 census.
No, but it does more closely match his age in the later censuses.
He is probably the same Charles Small as the one-year-old in Forfar in 1861.
I note that SP lists the a birth of a Charles Henderson in Rescobie in 1859. Looking at the International Genealogical Index, there is a birth of Charles Small or Henderson in 1859, father Leonard Small or Henderson, mother Ann Mackay.
So if Charles' marriage certificate says his mother was Ann Mackay, it would be reasonable to suppose that this is your Charles Small. So go on, get that marriage certificate.