I will be brief: to work in the spirit and idea of a Django app, it is ok for an app to import models from inside another app ? Say, a User statistics app will import models from a User app something like: from users.models import users
If you're building an internal app that has no chance of ever being released to the public, sure, do whatever you want.
If you're building an internal app that has little chance of ever being released to the public, but will possibly be used by future/current developers, sure, but be sure to document what the app needs to work properly.
If you're building an app for public release, try to keep it self-dependent (and django-internals dependent, ie, use what django provides, when possible). If you really need a third-party app to work, or if a third party app would make your code more manageable, then sure, include dependencies, but be doubly sure to document all requirements and necessary setup.
In most cases, you can do almost whatever you want so long as you have sufficient documentation.
I do, however, have to question the sanity of making your own User model that has the same name as django's builtin auth.User.