In Simonds Parish, Saint John Co., NB in the 1851 census, John Douglas is given as b. Scotland ca. 1797. A farmer, he arrived about 1833, and appears to live near Willow Grove in the Loch Lomond area. With him are his children, Thomas (1), Matilda (10), Margaret (21, m. John M. Roney (24)) and Robert Douglas (19.)
Anne, wife of John Douglas, b. Dumfriesshire, Scotland ca. 1795, d. Loch Lomond, Simonds Parish, Saint John Co., NB NOV1847 age 52 years.
In July of 1852, Robert and John Douglas, sons of Robert Douglas of Loch Lomond, set sail on the steamer 'Eastern City' to New York where they took the ship, 'Epaminodas' for Australia to the gold rush. Probably Robert, Sr. was Robert Douglas in 1851, b. Ireland ca. 1806, arrived 1830, also a widower. This would suggest they were unrelated, but also suggests that the draw of gold was felt in the area at the time.
On the other hand, Robert, Sr. may have been the John Douglas in 1851 as Matilda Agnes Douglas, 2nd d/o Robert Douglas of Loch Lomond, m. All Saint's Church, Loch Lomond, Simonds Parish, Saint John Co., NB by the Rev. J. W. Disbrow 15SEP1857 John Smith. As far as I know, there was, in 1851, only one Matilda Douglas; and she was John's daughter. This Robert Douglas appears to have been still living when, at his home in Loch Lomond, his granddaughter, Bessie Tyrell Smith d. 10AUG1864 age 2 years. She was the d/o John D. and Matilda A. Smith of the City of Saint John.
A Robert Douglas of Simonds Parish m. Upham Parish, Kings Co., NB 11NOV1854 Sarah DeBow, eldest d/o William DeBow (and of Elizabeth McDonald.) Apparently the ‘Sara’ Douglas, age 60, in the 1881 census of Simonds Parish, Saint John Co., NB with her son, William, and daughter, Elizabeth A. William was Presbyterian, but Sarah and Elizabeth were Church of England. If this was the same, then two of her children were:
241. WILLIAM DOUGLAS b. ca. 1841
242. ELIZABETH A. DOUGLAS b. ca. 1858
One difficulty you may be having is the loss of at least some of the census returns for 1851 and 1861 in the City of Saint John. Young men, in Loch Lomond, would seek employment there and could easily go 'missing'. The other problem is the number of persons surnamed Douglas in the general area. I rather think that Robert Bruce Douglas, who is a farmer in Loch Lomond in 1865, may be the Robert age 19 years in 1851; but that would be difficult to prove. There were also Robert Douglas, a justice-of-the-peace. I think he was the Robert Douglas father of Matilda Agnes (Douglas) Smith as in her marriage announcement, her father was Robert Douglas, Esq.; generally suggesting someone of some position in the Law. There was also a William W. Douglas, a farmer. (To carry speculation a bit further than one should, if Robert Bruce Douglas represents Scottish patriotism,then one might suspect William to be William Wallace Douglas. In fact, I would guess him to be the William Wallace Douglas of Loch Lomond, b. ca. 1839, s/o Robert and Agnes Douglas, who m. 26OCT1898 Mary Jane Whoolton (b. Golden Grove, Simonds Parish ca. 1848), d/o William and Margaret Whoolton. Interestingly, there does not appear to be a William b. ca. 1839 in the 1851 census of Simonds Parish.)
Robert Bruce Douglas is likely the R. B. Douglass who m. 3NOV1864 Jane McAdoo; both of Simonds Parish. I would also think he was the Robert B. Douglas b. ca. 1830 in Simonds Parish in 1881. His wife is Jane (ca. 1846) and children are Matilda A. (ca. 1866, likely the Matilda Agnes Douglas who later m. 25AUG1887 Edward C. Johnston), Elizabeth (ca. 1867), Ellie (ca. 1876) and Robert J. (ca. 1871.) Next door to Robert, in 1881, was William Douglas (ca. 1841), possibly the William Wallace Douglas as the latter, when married, was a bachelor in 1898 and this William is also unmarried, although there is a widow, Sara Douglas (ca. 1821) and possibly William's sister, Elizabeth A. Douglas (ca. 1858) living with him. On the other side of William is Robert F. Douglas (ca. 1854) with a wife, Anny or Amy (ca. 1846) and a daughter, Elizabeth (ca. 1880.) One might think that these three families, living in close proximity, were connected. Robert Bruce Douglas b. ca. 1830 in 1881, could well be the Robert Douglas b. ca. 1832 in 1851, s/o of John Douglas. Since both seem to have daughters, Matilda Agnes Douglas, this seems worth considering.
Robert Bruce Douglas and Janet McAdoo also had a son, Robert Bruce Douglas (b. ca. 1882.) Robert Bruce Douglas, Sr. d. Saint John Co., NB 19JUN1918 age 88 years. William W. Douglas d. Saint John Co., NB 1JUL1925. It is a bit surprising when, in 1891, Robert B. (62) and Jane (52) do not have a son, Robert (b. ca. 1882.) Perhaps he lied about his age when he married in 1920, but he certainly gave his parents as Robert Bruce and Jane (McAdoo) Douglas. Their children in 1891 in Simonds Parish, were Lizzie (21), Robt. Jr. (19) and Marion (14.) When Lizzie A. Douglas (b. ca. 1871) married Eugene Dewey Ring in 1892, her parents were Robt B. and Jane Douglas, so this seems to be the right family. Nellie Marion Douglas (b. Simonds Parish ca. 1876) also gave her parents as Robert B. and Jane Douglas when she m. 26APR1899 James A. Winchester.
In 1911 at Beaver Lake, Simonds Parish, live Robert B. (JAN1830), his wife, Jane (FEB1841) ad theirs son, Robert B., Jr. (AUG1875, so perhaps he was a bit older than 38 years in 1920 when he married.) Next door lives their daughter, Nellie M. Winchester (FEB1876, 22FEB1876 in 1901) her husband and children.
I had a bit of trouble finding Robert B. 'Duglas' in Simonds Parish in 1901 (b. 7JAN1830), with a wife, Jane (b. 10JUN1836) and son, Robert B. (16AUG1870; of the ever-fluctuating birth-date.)
The only Robert b. ca. 1830 in 1851 is the same b. ca. 1832, s/o the John Douglas you mention. Matilda also seems to be the same you mention, and Robert Bruce Douglas seems to name a daughter after her. Robert B., however, seems pretty clearly the s/o Robert and Anne/Agnes Douglas. This makes me suspect that the John, in 1851, is later Robert. This is conjectural, and you may be in a position to judge more clearly.