After you’ve create page with Portfolio List Page, this page will occurs on Choose your portfolio parent option when you edit or create new portfolio item. And you have to choose one of your portfolio parent as well for each your portfolio item. For more detail you can take a look right here http://support.jegtheme.com/documentation/portfolio/
Actually at the moment we don’t have those kind of feature, but you can assign certain portfolio item that you want to appear on those page. You just need to edit your portfolio item that have same category and reassign to same portfolio parent.