It was a "hearing" in Space Seed, not a court martial, and Khan was a civilian anyway. Different processes. Per The Menagerie:Perhaps Starfleet demanded a representative be present because of the supposedly unprecedented nature of prosecuting a Star Ship Captain. Most of the time we see a trial board of three and this one has four. Did Khan/McGiver's hearing have four members?
KIRK: A mutiny requires a trial board of no less than three command officers.
So, three at minimum and four as standard might be a rule-of-hand for a court martial. Also, "command" rank is Captain or above. Seems fairly simple and to-the-point.STONE: This court is now in session. I have appointed as members of this court Space Command Representative Lindstrom, Starship Captains Krasnovsky and Chandra.
Commodore Stone said he appointed Space Command Representative Lindstrom, so, I'm now thinking that Lindstrom is probably a high seniority Captain rank (now in a staff job) and was probably the second highest rank command officer (behind Stone) on Starbase 11. The other court members are passing Starship Captains would happen to be in port. Based on all their apparent ages and serious nature of the court martial, I assume they too would be the senior most Captains on station. They may also be the only two on station based on the later availability of Starships at Starbase 11 during The Menagerie when all they could muster was a damn shuttlecraft.