<blockquote> In the first case you would write "Estas seguro" for
males and "Estas segura" for females.
</blockquote>
Yes, ofcourse, you are right.
Quick question: what does Spanish MS Windows say? For instance, when you try to delete a file. Or when it confirms any action.
From what I remember, when you are referring to a mixed crowd of males and females, you use "ellos". So, the masculine form becomes the gender neutral default, so to speak. I guess I assumed this was also the case when you are addressing a single person of unknown gender. But I guess not.