Yes, it's possible use a custom template for mobile layout.
In the main control panel under 'Plugins settings' it's available a dropdown showing all the Joomla templates installed and avaiable.
You can choose the preferred template to be used when mobile devices are detected.
Be careful that choosing a custom mobile template several advanced features of Responsivizer built on 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 by other vendors.