How can I add extra groups to Finder's recognised kinds
I would recommend creating several saved searches, each for a specific file extension within a given folder, and then add each saved search to the Finder sidebar. For example:


In Finder, go to your working folder, then type something (e.g., xxx) in the Search field.
This reveals the Search row; click the '+' at the far right.
Change Kind to File extension, and type one of the extensions you need.
Remove the "xxx" or whatever you had originally typed.
Click the Save button.
Type a meaningful name for your search (e.g., Documents - PDF), and tick the Add To Sidebar checkbox.


Now that you have a saved search in the Finder sidebar, you can make duplicates of it.


In the Finder, right-click the Documents - PDF (that you just created), and choose Show in Enclosing Folder.
Duplicate your saved search, and rename it to something like Documents - DOC (i.e., for whatever you intend it to represent).
Select the duplicate search, and choose File menu >> Add To Sidebar.
In the Finder sidebar, click the Documents - DOC shortcut you just created.
Then right-click the shortcut and choose Show Search Criteria.
At this point, change PDF to DOC. Optionally, you could click '+' and add DOCX here, as well.


Unfortunately, you can't modify the folder location that a saved search points to; this seems like an oversight, but there you go.