Saying we "agree on 519 of 520 and disagree on 1" is a non-statement. The first person's belief in that 1 by definition precludes his belief in the other 519.
The point of the argument is to challenge people to question whether the existence of their god is any more plausible than any of the other thousands of deities whose existence has been postulated.
It works well against Christians or anyone else whose god has specific and well-defined properties. It doesn't work against deists since their "god" is sufficiently poorly-defined to be more plausible than Thor.
No it doesn't.
It amazes me how often I hear Christians say "this argument works well against atheists" and then make an utterly stupid argument that none of the atheists I know would take seriously. This strikes me as much the same -- it's an argument that atheists think should work well against Christians, but that most Christians will think is stupid and misses the point.
Contemporary thinkers might argue that god is well-defined, but ineffable.
I think what you are saying is that a Christian (for example) would say, "of course I don't believe in other god X, because I'm not allowed to". The 519 of 520 argument depends on whether the monotheist can also reject those other gods on their own merits.