If it really is just one CSS file, then doing something like this in the template would work. You'd need CSS files named the same as the URLs of the top-level pages

For example, if you're page is /company/, then you'd need to create company.css

<% control Level(1) %>
<link rel="stylesheet" href="mysite/css/{$URLSegment}.css" />
<% end_control %>

Hiding a page-type in the back-end can be defined by creating a canCreate() method on the appropriate class.

To disable the built-in classes, do it this way: since all the built-in types are sub-classes of Page, you could define this method on class Page: this would mean you could only create pages of type Page.

function canCreate() {
return $this->class == "Page";
}