There's already a mod for Inquisitors
here. Like Rodman49 said, if you add those to a game, you can bump up the percentage a bit higher. I'd even go as high as 75%, then, so that you can't possibly hit it by accident. The problem, of course, is that you'd pretty much only get that if you occupied the majority of the planet and purged the other religions from your own cities, which would almost definitely trigger the Domination victory.
But honestly, I've never liked the Domination victory, even though I understand why they added it. Just having the population shouldn't be enough, you should have to DO something with it, like concentrate on diplomacy (U.N.), religion (this one), or military (conquest); sure, once you hit a certain point a win is guaranteed, but I dislike how the Domination sneaks up on you. So, if you were to disable the Domination win and add more things like this, I think it'd be much better.