Here's more on your family from French Archives.

Yves COUFFON married Marie PLAPOUS in the 1820s or early 1830s. I have not found their marriage record yet. They had the following children born at Ploëzal, FR:

1-Marie-Yvonne, born Ploëzal 14 Feb 1834
2-Achille (twin), born Ploëzal 23 Mar 1836
3-Jean-Marie (twin), born Ploëzal 23 Mar 1836; died Ploëzal 11 Apr 1836 (16d)
4-Guillaume, born Ploëzal 11 Mar 1838
5-Marie-Louise, born Ploëzal14 Apr 1841

Achille COUFFON (#2 above) married at Pleudaniel, FR 12 Jan 1862 to Marie-Yvonne LE FLOCH d/o Pierre & Anne Le Pellec; Marie-Yvonne was born in Pleudaniel, FR on 22 Mar 1825, and was 11 years older than her husband.

Their Children born at Pleudaniel were:

1-Marie-Françoise, born Pleudaniel 10 Apr 1863
2-Jean-Marie, born Pleudaniel 8 May 1864
3-Achille-Marie (Your Ancestor at St-Pierre), born Pleudaniel 3 May 1865
4-Marie-Louise, born Pleudaniel 7 Jun 1867
5-Simon-Marie (twin), born Pleudaniel 2 Feb 1869 (also at St-Pierre)
6-Pierre-Marie (twin), born Pleudaniel 2 Feb 1869

Pierre LE FLOCH s/o Jean & Françoise Michel
born Pleudaniel 21 Brumaire Yr. IX (12 Nov 1800)
Anne LE PELLEC d/o Olivier & Francoise Richard
born Pleudaniel 2 Floreal Yr. II (21 Apr 1794)
m Pleudaniel, FR 24 Jan 1824

That's all I could find.

Attached some documents
Achille Couffon b 1865
Achille Couffon & Yvonne LeFloch m 1862
Pierre LeFloch & Anne LePellec m 1824