There are several reasons you may not be able to find your ancestor in earlier census records:
a) He MAY have had another given name or another 'dit' name. (nickname)
b) He may have been missed, mis-spelled, or listed by another surname.
Although it is important to be able to find ancestors in census records, death records probably are the first resource that should be checked, when available.
1. Death records
Silicour's death record MAY name his parents.
A 'Joseph S. Duplessis' died in Plaquemines parish at age 87. This implies he was born about 1854.
Could the 'S.' stand for Selicour?
You can order his death certificate from the Louisiana State Archives for a nominal fee.
His son Louis' death record may name his mother (Selicour's wife). If not, then check the death records of his other siblings.
The Louisiana Statewide Death Index has listings for a Louis and a Robert Duplessis. This Louis was born about 1883 and died in Plaquemines parish in 1931. Robert died in 1933 (Orleans parish?) and was born about 1875! [Contact me if you do not have this information.]
2. Marriage records
Selicour's marriage record would have his wife's given and maiden name, as well as their parent's names.
His children's marriage records may have the name of their father AS WELL AS their mother's maiden name.
[Hopefully, marriage records are intact.]
3. Church Records
Often, church records name other family members. This allows you to connect your direct line to others, who may be easier to trace.
Next, understand the history of the area in general. Who were the other families with your surname?
How were they related?
One Duplessis (Martin), was of French ancestry and moved from Orleans parish into Plaquemines.
Were your Duplessis ancestors associated with this early Plaquemines resident?
Court records may provide the answer.
4. Court Records
Did your ancestors own land? Check for succession (probate) records, which could list your Selicour(t) and other family members.
Are they listed in any court cases? Perhaps they testified, or were parties in court proceedings.
Once you have more family history, you may be able to find your ancestors in pre-Civil war Louisiana.
Although they may have been free people of color, court records will have to be searched to determine WHEN they were freed.
Did they come to Louisiana with the thousands that came from Haiti?
Were they 'manumitted'? Check for manumission documents or wills.
Per George W. Cable's historic book on LA Creoles, Duplessis was one of the old/original Creole surnames.
As such, they must have left quite a trail, including City Directories, early census records, Military and Notary records.
In the 1850 census, a 56 year old Mulatto, Widow Cyp Duplessis (born 1790's in New Orleans) is listed in Plaquemines parish. Also listed is 45 year old Casimir Duplessis, Mulatto, born in Louisiana (about 1805).
In 1830 there are six heads of household with free people of color in Plaquemines parish alone.
In the 1805 NO City Directory, Madam Duplessis was listed as head of a household with several free people of color.
Finally, NETWORK. Attempt to find others that are researching the same Surname in the same area.
If you contact one of the associations for Louisiana Creoles, you may find other Duplessis researchers.
I hope I have helped just a bit.
Feel free to post again if you need more information on any area I have (or have not) covered.