Is it possible to enable a checkout pane if the cart contains a certain product type?
Alternatively is possible to add a field to the cart page which requires to be checked to be able to proceed to checkout but only show and require this for certain product types?
You may want to follow
http://drupal.org/node/1122386
It develops Rules based pane visibility.
That should give you the possibility to enable a certain checkout pane when a situation meets your requirement.
Not sure what you're trying to accomplish with the second question, but have a look at these modules:
http://drupal.org/project/commerce_extra_panes
http://drupal.org/project/commerce_custom_product
they might meet your need.
I'd do a 'terms of service' like pane based on a Rule that checks for a product type. (commerce_extra_panes module + the issue)