I am wondering if there is a way to setup a forum category, have some forums under it BUT not show it in the main forum index and instead link to this category from the main menu.

 

 

If you don't care that it exists in the browser source html you can use custom css to make it not show:

body[data-pagemodule="forums"][data-pagecontroller="index"] [data-categoryid="11"] {display:none};