Yes, it's possible to use a custom template for the mobile layout.
In the main control panel under 'Plugins settings' it's available a dropdown showing all the Joomla templates installed and avaiable to be selected.
You can choose the preferred template to be used when mobile devices are detected.
Be careful that choosing a custom third party mobile template, several advanced features of Responsivizer built on the proprietary template framework will be lost.
Exclusive features such as module exclusions, easy template styling, etc won't be possible using a third party template provided by other vendors.