Well, I can refer to several books that I read about history of photography in the Ottoman Empire, but all are in Turkish I'm afraid. Even if they were in English, I doubt you would be satisfied until every person in every photo is proven beyond reasonable doubt to not to be a zeybek; confirmation bias is tricky like that.
I can't disprove those were not zeybeks, but keep in mind, those zeybeks were outlaws who could not easily walk to a big city photo studio with all their weapons for their photos to be taken. Not to mention, since the usual way to become a zeybek was either running from the law for murdering someone in their "civilian" life, or deserting from the army(both crimes punishable by hanging) those people wouldn't prefer to let their faces to be known by authorities. Actually most of them lived in mountains and rarely even go down to rural villages because they would be risking their lives, (being ambushed by soldiers or rival gangs, or even villagers themselves) every time they leave those mountains. There are some photos of famous zeybek leaders(Efe) but those were taken either after they were arrested, or pardoned(for volunteering to join Ottoman army with all of their forces for war).
So, as you say, using our judgement, which possibility is more likely?