Send an Email When Tag is Applied - or When Added Automatically to a Smart Group
You should be able to do what you're describing using CiviRules.  However, to be safe, I would start by testing with an action other than sending email - or at least do it on a test server that redirects emails to the logs rather than sending live emails. 

Here's a screenshot that shows how to set up what you're describing.  Note the trigger ("Tag added to entity") and linked condition ("Contact has tag").