Is adding a new entity on the new version of my core data model going to crash the apps using the older versions of core data model?
What if this entity has a relationship with an existing entity.
oldEntity (1)------------->(oo) newEntity
Sounds like a simple case for versioning, which can be done quite simply in a lot of instances. This sounds like one of them.
Check out the docs here: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreDataVersioning/Introduction/Introduction.html