I suggest that only create highpoly model in Zbrush, Decimate it and export if for blender.
Cause, any low poly mesh, which you create in Zbrush, will be un-usable in blender due to Triangles. Also, it will be a problem to animate the Zbrush Low Poly (which is very high, in terms of polygon, as far as blender/game engine is concerned). So, you need to create your lowpoly model in Blender.
Since you are starting in 3D development, its better to invest your time learning blender for modeling and sculpting, instead of trying to learn 2 different applications.
Blender is also very suitable for game modeling and has better asset pipeline when it comes to exporting for unity.
To learn more about Blender. Visit www.blendercookie.com and for unity www.unitycookie.com