To use projection textures I turned off the Zadd and changed to RGB channel and Colorize in the Polypaint menu. Then I simply used the textures in the lightbox and painted them directly onto the subtools. In some areas, where I wanted darker or lighter effects, I painted by hand. I also used the “new from polypaint” under the Texture Map tab, to get the textures onto the right uv map.
The modeling process; I imported my low-res mesh into Zbrush and divided it into Subtools using the “Groups Split” option. Every Subtool has a division level of 5-7 for a good, smooth result. I found the alpha brushes essential during the sculpting, to make small details.
Furthermore I tried exporting a displacement map directly from Zbrush, but it seems to result in a bugged map, so I will try using Xnormal instead and see if that works better.