The short answer is that there was pretty much a "carnival atmosphere" prior to Manassas and Shiloh. No one thought that the war would last, pretty much most southerners thought that the northern soldiers would run at the mere sight of the Confederate army. Many people had uniforms made, and a lot of soldiers had photographs made. There was never a time that uniforms were "standardized", but in the early days while units were forming and training up was when most really fancy uniforms, such as the ones you describe in the photo, were seen.

Without knowing the situation or seeing the photograph, I'd say that the uniforms were some that the family had made by a local tailor, and the photograph was made before the 2nd Mississippi left for the east. They could have been officers, but not necessarily, pistols and swords, that infantry privates never carried on the battlefield, were often seen in these early war photographs.