Can Email Manager coexist with Harmon.ie

Yes, Email Manager can be deployed on the same SharePoint system where Harmon.ie is used.

The existing email metadata columns from Harmon.ie can also be used with Email Manager. Email Manager also supports combining user names with user email addresses. For example,
Paul de Jong <[e-mail]>

Uploading emails via Email Manager will detect if an email already exists and show a dialog similar to Harmon.ie to allow users to add the email as a new document version, append a unique sequence number like (1) or cancel the upload request.