The "Interpretive Specialist" title is one used by the State of Kentucky, and is basically someone who interprets first person documents, archeological findings, and terrain features in relation to the site. Kind of a assistant historian, or glorified tour guide, as the Perryville Battlefield does not have or use official Park Rangers. I have been here at the Perryville Battlefield for the past ten years studying this battle directly under the Park manager.
Yes the 21st MI was brigaded with the 36th ILL under Col. Nicholas Greusel, along with the 88th ILL and the 24th WI. Greusels' bgde was engaged six miles west of Perryville by Joseph Wheelers' force on Oct 7th near the village of Pottsville. On Oct. 8th the bgde defended Peters Hill against Col. Sam Powells CS bgde. Being out of their proper position at the time, the 36th ILL bore the brunt of the attack and incurred the most casualties of the brigade [12KIA, 61WIA, 4MIA]. Your casualty would likely have received his artillery wound curtesy of Capt. Overton W. Barrett's Missouri Battery.
Again, Peters Hill is some distance [2 miles] from the Goodnight farmstead, and the prospect of identifying the Union troops that guarded the post battle CS hospital at that location intrigues me.