Trusts should use what characters are there, and also summon a number of generic light warriors like Elidibus did in his fight.
It's getting pretty obvious when they just need more people there to fill out trusts, and I'd prefer they not write themselves into a corner like that too much more. Y'shtola, G'raha, and Estinien show up just in time for the lvl 99 trial, then they split off from the group immediately after.
We joked about how Thancred and G'raha can only tank because they needed to fill out the tank trust.
I'm all for the trust system even though I'd never want to use it. If the game starts to die out again, you're not going to want to spend 2 hours in Party Finder because Duty finder isn't popping for a story dungeon. It just shouldn't bleed into the actual story itself.