What are the limitations of using Django nonrel with Google App Engine?
The big limitation is that the datastore doesn't do JOINs, so anything that uses JOINS, like many-to-many relations won't work.
Any packages/middleware that uses many-to-many won't work, but others will.
For example, the sessions/auth middleware will work.  But if you use permissions with auth, it won't.  If you use the admin pages for auth, they use permissions, so you'll have some trouble with those too.
i8n works.
forms work.
nonrel does not work with ndb.
I don't know what you mean by "until your project gets bigger".  django-nonrel won't help with the size of your app.
In my opinion there's two big reasons to use nonrel:

You're non-committal about App Engine.  Nonrel potentially allows you to move to MongoDB as a backend.
You want to use django packages for "free".  For example, I used tastypie for a REST API, and django-social-auth to get OAuth for FB/Twitter logins with very little effort.  (On the flip side, with 1.7.0, they've addressed the REST API with endpoints)
I understand that full django can be used out of the box with CloudSQL. But I'm interested in using HRD. I'd like to learn more about what percentage of django can be used with nonrel. Does middleware work? How about other features of the framework like i18n, forms, etc. Also does nonrel work with NDB?
The background here is that I've even using webapp2 and before that webapp and find them great until your project gets bigger. So for this project I'm interested to reevaluate other options.