Your ancestor, James Dallas McDonald, was living in Marion County, Mississippi on the 1850 census, but on the 1860 census he was living in Livingston Parish, Louisiana.
The McDonald family appears on the 1850 census of Marion County, Mississippi as follows:
Name: Wm Mc Donald
Age: 66
Birth Year: abt 1784
Birthplace: Nova Scotia
Home in 1850: Marion, Mississippi
Name-----------------Age
Wm Mc Donald------66------male
B A Mc Donald------45------female-----(Ann)
C Mc Donald--------19-------male------(Calvin)
Wm Mc Donald-----17-------male------(William)
M Mc Donald--------14------female----(this may be Mary Whitehead)
R Mc Donald--------11-------male------(Rudolph)
S Mc Donald---------9-------female
Dallas Mc Donald----5-------male------(James Dallas)
Flora Mc Donald-----2-------female
Source Citation: Year: 1850; Census Place: , Marion, Mississippi; Roll: M432_376; Page: 189A; Image: 380.
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The McDonald family appears on the 1860 census of Livingston Parish, Louisiana, all on the same page in the following order:
Name: Wm Mcdonald
Age in 1860: 27
Birth Year: abt 1833
Birthplace: Mississippi
Home in 1860: Livingston, Louisiana
Post Office: Springfield
Name------------------Age
Wm Mcdonald--------27
Sarah E Mcdonald---23
Thos Mcdonald--------3
James Mcdonald-----10/12
Source Citation: Year: 1860; Census Place: , Livingston, Louisiana; Roll: M653_413; Page: 249; Image: 248; Family History Library Film: 803413.
Name: Rodolph Mcarnal
Age in 1860: 23
Birth Year: abt 1837
Birthplace: Mississippi
Home in 1860: Livingston, Louisiana
Post Office: Springfield
Name------------------Age
Rodolph Mcarnal-----23
Ellen Mcarnal---------18
Ann Mcarnal----------54------(mother)
Dallas Mcarnal-------15-------(James Dallas McDonald)
Source Citation: Year: 1860; Census Place: , Livingston, Louisiana; Roll: M653_413; Page: 249; Image: 248; Family History Library Film: 803413.
Name: Mary Whitehead
Age in 1860: 25
Birth Year: abt 1835
Birthplace: Mississippi
Home in 1860: Livingston, Louisiana
Post Office: Springfield
Name---------------------Age
Joseph Whitehead------25
Mary Whitehead--------25
Susan Whitehead--------1
Source Citation: Year: 1860; Census Place: , Livingston, Louisiana; Roll: M653_413; Page: 249; Image: 248; Family History Library Film: 803413.
Name: Calvin Mcdonald
Age in 1860: 30
Birth Year: abt 1830
Birthplace: Mississippi
Home in 1860: Livingston, Louisiana
Post Office: Springfield
Name---------------------Age
Calvin Mcdonald---------30
Delphine Mcdonald------17
Source Citation: Year: 1860; Census Place: , Livingston, Louisiana; Roll: M653_413; Page: 249; Image: 248; Family History Library Film: 803413.
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John R. Stafford also appears on the same page of the above 1860 census living on the other side of William McDonald. It is my understanding that John R. Stafford is also one of your ancestors.
I would suggest looking for James Dallas McDonald in a regiment organized in Louisiana.
Booth's Records of Louisiana Confederate Soldiers is available online at the following link:
http://www.usgwarchives.net/la/military/wbts.htm
James Dallas McDonald did not apply to the state of Louisiana for a pension, nor his widow, and there is no reference to military service on his gravestone.