The primary source for Mississippi units in the Civil War (and earlier) is Dunbar Rowland's "Military History of Mississippi, 1803-1898". Rowland compiled a unit history of each regiment and most of the histories include the commanders and staff officers as well as the name of each company, the county they were formed and the company commanders.
For 28th Mississippi Infantry Regiment, it has "See cavalry regiments".
For the 28th Regiment Cavalry, there is a 4-1/2 page history with a list of officers and company commanders.
Commander was Colonel Peter B. Starke. The first lines of the history states:
"This regiment was organized early in 1862, for three years or the war, extra to the requisitions upon the State for infantry. It is numbered consecutively with the infantry regiments." This regiment was began active duty in response to the Union naval attacks on Vicksburg in May 1862 (not 1863).
Company G was commanded by Captain James M. Smith. Most of the companies have a specific county where the men were recruited from but Company G does not identify with any county.