Mezzanine and django_tinymce have nothing to do with each other - presumably you're configuring software that isn't even installed.
From the Mezzanine docs:
If you’d only like to customize the TinyMCE options specified in its JavaScript setup, you can do so via the TINYMCE_SETUP_JS setting which lets you specify the URL to your own TinyMCE setup JavaScript file.
http://mezzanine.jupo.org/docs/admin-customization.html#wysiwyg-editor
Not quite as straight-forward as django-tinymce, but you can get to the same result.
Also note that the styles (such as the tiny font mentioned) have since been updated in the development version of Mezzanine, so if you build against that you might not even need to configure things:
https://github.com/stephenmcd/mezzanine/commit/6dff64bf058ac0e83c3c313b4167f8bbd1ac9560