I don't have any answers for you, but I think this will add to the confusion.
I'm pretty sure there are two listings for this family in 1870.The one you posted for Lauderdale Co. and one in Noxubee Co.In the second listing Lucy is "Pelina" but the other info is pretty much the same, Josephine, 7, Emma, 3, Mary E. SALLS, 23, Serena SALLS, 14.There's also a Jennie DUBLIN, 23, and Theodosia DUBLIN, 6.
If you look at the image for the Lauderdale listing Susanna I think is really Surena.
Thinking that Mary E. and Serena might somehow be related to Lucy/Pelina, I found them in the 1860 census in Neshoba Co, MS: SAULS, Fransis age 37 b. NC Sarah A. age 18 b. AL Mary E. age 16 b. MS Malissa age 13 b. MS Wm. age 11 b. AL Henry C. age 9 b. TX Sarena age 4 b. TX
I was hoping to find them in 1850 to see if Lucy/Pelina was an older daughter, but I can not find them.