The question is what do you mean by combatant.
To my current knowledge of the game all lords can actually be combatant, they do not cause of the current state of the game and how clans are handled.
Basically in a clan you have a max number of parties, and so a max number of commanders. Commanders accessed via `Clan.CommanderHeroes` (it's an array) are the heroes you will see on the map leading a party,.
Now that do not means non commanders can't fight, in fact they could join a party and fight. An example is when you marry the daughter of a lord, she is non commander most of the time and once she get in your party, she will be a companions. (And yes you can make her lead an army then but it's because player have mechanics the AI do not)
So why they don't join is simply cause the AI do not manage companions so far. (And that's why i'm working on a mod expending companions use for both AI and player).
Still there are some other tags (or Boolean) that might prevent a lord to fight, like if he is a child or not. I could list a few ... isChild, isActive, isAlive, isDead, isDisabled (Yeah you have one for being alive and be dead and one for active and disabled ... I'm not the one who wrote the code!)