Django models with external DBs
I would create the minimal django models on the external databases => those that interact with your code:
Several outcomes to this

If parts of the database you're not interested in change, it won't have an impact on your app.
If the external models your using change, you probably want to be aware of that as quickly as possible (your app is likely to break in that case too).
All the relational databases queries in your code are handled by the same ORM.
I have a typical Django project with one primary database where I keep all the data I need.
Suppose there is another DB somewhere with some additional information. That DB isn't directly related to my Django project so let's assume I do not even have a control under it.
The problem is that I do ont know if I need to create and maintain a model for this external DB so I could use Django's ORM. Or maybe the best solution is to use raw SQL to fetch data from external DB and then use this ifo to filter data from primary DB using ORM, or directly in views.
The solution with creating a model seems to be quite ok but the fact that DB isn't a part of my project means I am not aware of possible schema changes and looks like it's a bad practice then.
So in the end if I have some external resources like DBs that are not related to but needed for my project should I:

Try to create django models for them
Use raw SQL to get info from external DB and then use it for filtering data from the primary DB with ORM as well as using data directly in views if needed
Use raw SQL both for a primary and an external DB where they intersect in app's logic