Can we have Editors be allowed to create or modify only certain types of artifacts (such as User Story) but not others?
Rally's permission model only associates users with workspaces and projects. Within a scope of a workspace and a project users can be granted Admin, Editor or Viewer permission. They can also have 'No Access'.
There isn't a way to select or specify permissions per artifact type. Editors will have the same permissions across all artifacts in that project.
This can certainly be posted as an enhancement request for a future Rally feature.