Well, I found the answer to the first part of my question tucked away in the tutorial:
The table must have a primary key, and may have zero or more unique keys. The primary key and each unique key may contain multiple columns.

Also, if you are desparate there is even a workaround for not having a primary key:
If the table has a multi-column primary key or does not use a column type that supports auto-generated values, you can define a custom primary key generator function using the primary_key_generator method of the Rose::DB::Object::Metadata-derived object that contains the metadata for this class.