He was in Co. A of the Spanish Regiment of the European Brigade. He was a private. Here is what Booth had on him:
Barretos, Pedro, Pvt. Co. A. 5th Regt. European Brig. (Spanish Regt.) La. Militia. On Roll dated New Orleans, La., April 2nd, 1862.
From my analysis of the data available to me, the Spanish Regiment started as the Spanish Rifle Regiment -- before changing its name to the Spanish Legion in spring 1861. This was considered a Home Guard unit under Gov. Moore's proclamation of late-summer 1861. It was eventually added to the European Brigade when that unit was formed in spring 1862, and was designated "5th Regiment (Spanish Regiment)" of the European Brigade. It was never in active PCSA service but was active to protect New orleans when Confederate forces departed the city in advance of Federal forces. Gen. Butler eventually disbanded the brigade (and the Spanish Regiment) after taking over New Orleans.