But even those are just guidelines. The idea is that Nat 20 helps you publish your stuff, with you getting involved at whatever level you want. If there is any predominating guideline, Nat 20's main aim is to avoid losing money on a given project - there's no capital behind the company, so if we think a product will make less money than it costs to produce, then we can't afford to do it.
One thing I can add is that usually we like to do a little work with a person before we agree to actually work on a project. Since ENWorld produces Asgard magazine every 2 months (though the current issue is a month late, sorry), the perfect avenue for us to get experience working together would be to pitch an article for Asgard. Then, if things work out well on that article, we can move along toward whatever product you actually want published.
Of course, if some big name like Gary Gygax, Bruce Cordell, or St. Thomas Aquinas (hey, it could happen) wanted to write for us, we'd probably be able to trust that they're proficient writers.