In the admin I have as an option when you create a new item to let you choose which categories it belongs to through a TreeMultiselectField.

$treeMenu = new TreeMultiselectField('Categories', 'What categories does this belong in?', 'PortfolioCategory');

PortfolioCategory is a class that extends Page.

Now, when you click on the drop down to select a category, it just shows the entire site tree. Is there a way to customize it to say, only show Pages of type "PortfolioCategory"?

Also, is there a way to have an option in the drop down automatically selected by default? Ideally, when you click "Add New Item", the drop down is there and one of the options is already selected and cannot be unselected.