Hi, I was using Mosaic in Plone 5.08 and worked as advertised. When I went to Plone 5.1RC1 I found I needed a new buildout configuration. Not sure if anyone else has encountered this. However, in case you have trouble with mosaic in Plone 5.1RC1, here is the place that gives the buildout that works.
Thanks to the creators or Mosaic, it allows me to easily customize a page and create templates