I didn’t understood exactly what the problem is but if you have a page with two or more objects of different types and each type (xclass) has a different sheet associated, then only one of the sheets will be used (the first one found) to display that page. So if the object you’re adding with the event listener on every page has a sheet associated then it will break the pages that already have a sheet (because they have already an object with a sheet associated). In this case you should remove the sheet associated from the object you add with the event listener and manage the display from the skin or from an UI extension point [1] (I assume you want to display something related to this object on each page).
Hope this helps,
Marius
[1] http://extensions.xwiki.org/xwiki/bin/view/Extension/UIExtension+Module