I'm gonna wait for Steam version
Free work is ok if it's not-commercical project, such as modding for game. If it's commercical and they're going to make money off your work then it's unfair.
CG3DANK has a thorough tutorial on sculpting rocks in ZBrush for you to check out. The guide covers the full process, shows different useful brushes and discusses the creation of presets in form of alpha.
In this tutorial, I am telling you the full procedure of rock sculpting step by step,specific brushes used for sculpting and how to create preset of rocks in form of alpha.
The channel is full of other useful tutorials, so make sure to check it out for more videos on ZBrush, Blender, Mudbox, 3ds max and other popular tools.