Ken,
I'm pretty sure that is the same man. However, there is a good bit of information missing and/or contradictory. Fold3 does not have a record of this man with either spelling - Barreau or Beneaux. However, your Stanislas Beneaux is shown on the NY Military Museum site listed on the roster of the 35th NY. It shows: "Age, 19 years. Enlisted, September 14, 1861, at Watertown, to serve three years; mustered in as private, unassigned, September 18, 1861; no further record." Unassigned meaning not assigned to a company. There is a Captain Henry D. Rich commanding Company E. Since the spelling of the last name could be all over the board, I checked Soldiers and Sailors for men with the first name of either Stanislas or Stanislaus, and came up with only 10 for the state of New York. None of them with a last name anything like these. Also, since he said he was later in the Maine Cavalry, I checked for men of that first name in Maine, and found none at all. I also checked the two spellings of the last name and came up with nothing. He also says that he was at Gettysburg, and he could have been, but not in the 35th New York. In May 1863, all the three year men were transferred to the 80th New York, and in June, what was left of the regiment (presumably 2 year men) was mustered out of service. The NY Military Museum roster of the 80th does not show him in that unit. The 80th was certainly at Gettysburg, suffering the majority of its casualties during the Pickett-Pettigrew-Trimble charge. Since Union records tend to be more complete, the lack of such made me suspicious of his claims, but what you found gives them a bit more weight. I'm not sure what to think now.