I don't think it is very likely that Thomas born in 1808 is the one who married in 1826.
First, it is extremely unusual for men to marry at age 18. Thomas must have been apprenticed to a sailmaker to learn his trade, and until he had completed his apprenticeship and established himself he would hardly have been able to support a wife and family.
Second, none of Thomas' family were named either James or Margaret. If Thomas and Charlotte followed the naming tradition, his parents' names would have been Thomas and Jane, and Charlotte's would have been David and Jessie/Janet. However you can't be absolutely certain of this because of the missing information.
The 1841 census lists Charlotte as 43. So if her age in that census is accurate she would have been born in 1796/1797. The 1851 census lists Charlotte as aged 55 and born in Craig. So if her age in that census is accurate, she would have been born in 1795/1796. A Charlotte Garden was born in Craig on 20 March 1797 and would have been 44 on census day 1841 and 54 on census day 1851. The parents of this Charlotte Garden were David Garden and Ann Kinnear. Have you found a death certificate for Charlotte? (She isn't the one who died in Montrose in 1868, aged 70. That one was born in St Cyrus in (allegedly) 1793, married John Milne in St Cyrus on 26 November 1819, and there is gravestone in Montrose commemorating her and her husband.)
I see that Thomas Jr was born on 26 January and baptised on 14 February 1830 in Ferry Port on Craig, mother's name given as Charolette Gairns.
Unfortunately the baptisms of Jane (1832/33), David (1835/36) and Charlotte (1837/38) don't seem to be in the index at Scotland's People or anywhere else reliable. So it's not possible to be 100% certain there are no missing children who died in infancy.
There is a Thomas Nicoll, son of Thomas Nicoll and Jean Chalmers, baptised in Dundee in 1789. Could this be your Thomas, perhaps? He would have been 37 in 1826, which is on the old side to be getting married for the first time, but maybe it wasn't a first marriage? A marriage at 37 is still more likely than at 18.
The only official document Thomas appears on is Jane Nicol(l) (Sharp) death cert where he is the informant of her death, despite his son still being alive.
But according to the census he had died by 1851, so he could not possibly have signed a death certificate in 1892.
Charlotte Nichol age 72 , 1874 , Montrose Angus , 312/10.
Charlotte Gardyne age 72 , 1874 , Montrose Angus , 312/10
I had Charlotte still alive at 73 yrs, co- habiting with a widower so will double check that.
Remember that Charlotte should have had a better idea of her own age than whoever registered her death.
In 1854/55 Thomas decided to add an L to the surname making it Nicoll.
Actually, he probably didn't. Spelling was a very inexact science - your name was spelled however the person writing it down thought it should be spelled. Note that in Jessie's baptism is appears as Nichol, even though the marriage and the baptism of Thomas Jr both use Nicol. Don't try to read anything into spelling.