You make it sound as if custom normals are something bad :) For the low-poly it doesn't matter at all if you bake from a mesh with custom normals or not.
Well, here's a quick solution that costs money https://cgcookiemarkets.com/blender/all-products/retopoflow/ and you support Blender development at the same time.
The manual usually has all the information you need on the operation of a node type: https://www.blender.org/manual/composite_nodes/types/color/z-combine.html
Poly: Don't bother with the official releases, they're outdated by day two of their release. Check out graphicall.com for constantly updated builds. Ofcourse these can (and will) contain bugs, but then every alpha or beta will. Also, to the guy saying he can't use 2.5 because it's so different from 1.49: really? Oh come…
@MmAaXx , @"another caveman" : FWIW I was just recently wondering about the very same thing (wouldn't it be cool if there was a way to preserve/set all normals as custom at the time of splitting !) ; but then found out that there is a very good alternative available. Basically all the functionality of the Data Transfer…