Is it going against best practice to have some items in the main navigation clickable and other items not clickable they just reveal a drop down? Should the pages that reveal a drop down have a landing page?
Yes it does. You need to make the drop-down navigation item clickable to make the link accessible to touch interface users.
You don't need not to have a landing page for the drop down-click, just make the drop down-item stay open when the top navigation have been clicked or hovered.
Off Topic: Here are some useful scripts for implementation: Drop-Down Menu: 30+ Useful Scripts To Enhance Header Navigation