I think the "St." part is a holdover from when people tried (and still do try) to tie Rene into the Pierre/Peter de St. Julien line, by making Rene identical to Peter/Pierre, Sr.'s son Peter/Pierre de St. Julien, Jr. who was born on 4 July 1669 at Vitre, France.
I don't believe anyone has ever found a record of Rene or any of his descendants where they used "St." in front of the name.
Paul R. Julian, "A Genealogy of the De St. Julien Family of South Carolina," *Transactions of the Huguenot Society of South Carolina* No. 105 (2001), 70-98, has a good article on the family of Pierre De St. Julien de Malacare, that shows that his lines all daughtered out, and he has no St. Julien descendants.
I noticed you also had a post on William FIELD(S) yesterday.Do you by chance have a picture of his tombstone?A relative was there a little over a year ago, and was unable to find it.