Command Focus is understandably mainly associated with the Edit window. The single keystroke shortcuts it offers can be a major upgrade for editors wanting to get faster. Unlike the Edit window, the Mix window only offers one command focus "a-z" button - in the groups list. Using this button mix groups can be selected and deselected using their ID letter. By default groups are assigned the ID letters alphabetically (with "!" reserved for the all group). However you can assign any letter you like in any order you like.
Use The First Letter Of The Name Of The Group As The Group Letter
Instead of having to remember that your drum group is "a" and your vocals is "f", why not use memorable letters which link to the content in some meaningful way? The example in the image above is an idealised version where no two groups are competing for the same letter but if, for example, I wanted to create a group for backing vocals and both B and V are taken I could use u for "back up" or o for "oohs" (if thats what the BV was doing) - you get the idea.
While it’s easier to assign letters as you create groups, with a little shuffling its possible to re-create groups and change their ID letter retrospectively. I use this all the time as an alternative to the "blunt instrument" alternative which I see used constantly - suspend all groups (Shift+CMD+G/Shift+Ctrl+G).
Although presented as a technique for the Mix window and mix groups, this technique works just as well for edit groups in the Edit window.