If their goals align with fash they can’t, that’s kind of the beauty of this relationship. Also gadara are personal relations not organisational ones in-universe, so it isn’t so much can tankies as a whole be trusted and more of a can this individual tankie be counted on to act against fash. Its also somewhat circumstance based, if I understand their motivations and ideals, I can probably determine if a given fash threat will be enough to keep their mind on the right target.