Hi Melody 

Thanks for reaching out to the community!  Unfortunately we can't do this with Team Folders.  We're always looking for ways to improve Lucidchart we would really value your feedback with a feature request via this form.  One way to share a mix of editable and view-only documents with your team is to create separate team folders each with unique sharing permissions.  Regarding your question about changing the sharing permission for individual documents within a team folder please refer to the Team Folder Ownership section within this Lucid Help Center article on Team Folders. 

You can grant greater access permissions to a user for an individual document or board than the user has for the Team Folder but you cannot restrict access to individual documents or boards if the user has greater permissions to the Team Folder. In other words if you have edit access to a Team Folder you cannot have view-only access to documents and boards in that Team Folder; you will be able to edit all documents and boards within the Team Folder. However if you have view-only access to the Team Folder then you are able to gain edit access to a document or board within that Team Folder.

I hope this helps!