I'm not sure this helps, but I usually draw and rig each character in their own project file. When the character is ready, I'll import it to the project with the environment where it will be animated.
On some productions, I've used the Reference option when importing the rig. This option maintains a connection between the original rig and the imported one. This means I can make certain changes to only the original rig, and the changes can be easily proliferated throughout other project files that contain the imported rig Reference.
Word of caution: The current Reference system has a bug that can activate all the channels in the rig and make a mess in the Timeline. When this happens, you have an option to either sync every channel individually or to break the Reference and use the rig as a normal 'original' rig. LM is aware of this bug, so hopefully we will see a fix for it in the future.
A good example is the cats I created for Boss Baby: Back In Business, which can be seen in my demo reel here...
Boss Baby Cats
In this case, I made a single cat rig. Then I imported four copies of the cat into a new project file as references, making sure to check on Unlink Shared Styles. This made sure each cat had its own unique set of Custom Styles so they could be easily recolored as individuals. After the cats got their individual looks, I imported this file into the scene files. Now, if I had to make a change to the rig, I only had to edit and save the original version of the rig, then apply the Update command to the file with the four referenced cats, and apply Update in the scene with the many cats. In reality, the workflow wasn't exactly as smooth as that may sound, but it still made the work easier than if every cat had been an individual rig. (That was a long time ago, and that's the best I could remember it.)
Another great example is this scene from Wolfwalkers, which has many, MANY characters, and they're all unique...
Wolfwalker scene with 163 characters
This shot is pretty crazy. I
think there's another video on the Moho Animation channel that breaks it down in greater detail.
Update: I found it. It's a scene walkthrough guided by Victor. Definitely worth watching...
Wolfwalkers: A walk through the 163 rigs file