1st Regiment, Mississippi Infantry (Percy's) (Army of 10,000)
"Units of the Confederate States Army" by Joseph H. Crute, Jr. contains no history for this unit.
Regiment Companies with the County of Origin
Men often enlisted in a company recruited in the counties where they lived though not always. After many battles, companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.