How to add active class to a specific menu only, while ignoring other menus?
Try the  following modules

1)Menu Position 


  Often times site builders want certain types of content to appear in a specific position in the navigational menu. The simplest solution, adding all of that content individually to the menu system, has performance and usability issues. (Imagine hundreds of menu items added to one spot in the menu.)
  
  This module allows for the creation of rules that will dynamically add the current page into the menu system at the requested spots.


I've used Menu Position module for one of my project and it works great!

OR

2) Menu Trail By Path


  Menu Trail By Path sets the active-trail on menu items according to the current url.
  
  For example if you are at yoursite.com/blog/category1/article1
  Menu Items with these paths will get the active-trail class on them and expand accordingly.
  
  blog
  
  blog/category1
  
  blog/category1/article1