Well, I don't know anything 'bout anything regarding Gaogaigar, and my knowledge of Zero no Tsukaima doesn't extend beyond the first season of the anime. Between that and the protagonist being a self-insertion, I'm disinclined to weigh in on the plot, considering how little I know of the series involved (thus the characters and where the story normally leads) and how there's no concern over being faithful to the protagonist's character (seeing as you should know yourself the best). But I felt I could still offer a little something.
On the technical side of things, the writing's fairly good and I don't feel like I have to gouge my eyes out trying to read it. It could use some touching up, though they're usually typos and the like. (For instance, using a comma instead of a period before you tug on your hair in the prologue). Beyond that, nothing really sticks out -- at a casual glance -- that I feel the need to pound down on with a mallet. Well, not that it matters, but Xellos' hair is a bob, not a bowl cut. ;p