In relation with multiple registry configs for .npmrc and gemfury I have experienced multiple challenges.

Looking through various guides there are many suggestions but also challenges with these.

I see one item missing from your guide in relation with registry config, as one can now install npmrc through npm, the following procedure is available:

npm install npmrc -g
npmrc

First run initializes ~/.npmrcs folder

Within the folder the ~/.npmrcs/default will be initialized and populated by your ~/.npmrc if that file exists, and/or replace that file with a default config.

adding more profile files to ~/.npmrcs folder will allow you to use:

juksefantomet@juksefantomet:~$ npmrc
Available npmrcs:

   default
   profile_juan
 * profile_two

additionally it allows you to use:

juksefantomet@juksefantomet:~$ npmrc profile_juan
Removing old .npmrc (profile_two)
Activating .npmrc "profile_juan"

This allows for a good structure where multiple configs can cause issues if not scoped and guide wise allows from a more visible assistance to the end-user.