Morgan, any and every spaghetti western is eligible. Stanton keeps a spreadsheet of every single spag which has been voted for. It's how the top fifty is compiled in the first place.
If a movie's not a spag, it's not eligible. But that's it, really. Oh, and every list submitted needs to be a full list of twenty, ranked in order of preference (so, you can't just submit a top ten for instance, or a twenty in no specific order) otherwise Stanton can't accurately award points to those movies.