Kent, I think this is the captain you are looking for, Captain James Gillaspie, spelled with an a.
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=Gillaspie&GSfn=j&GSby=1850&GSbyrel=before&GSdyrel=all&GSst=46&GScnty=2755&GScntry=4&GSob=n&GRid=30560891&df=all&

My great-grandfather served under Gillaspie in Company E, 9th Nichols'.

But prior to taking the company of men from Huntsville to Galveston for service under Nichols, Gillaspie was captain of a company of Home Guard, also called Texas State Troops, or Texas Militia. The Texas militia or home guard was a training ground for young soldiers who would go from the home unit to the Confederate States Army. But when the young men left for the battlle front, the older and infirm men continued serving in the home guard, the Texas State Troops. They were the first line of defense against hostile Indians and incursions from the Mexican border, and were subject to being called up by the governor in case of emergency.

If you would go to the Texas State Archives and ask to see the correspondence files of the 17th Brigade, Texas State Troops (Montgomery-Grimes-Walker), you would likely find McShan on a militia list. These lists are unpublished and unscanned and are available only to visitors to the archives. They may be photocopied.

Good luck!