Dear @Fgeser,
For the source routine you might also have a look at the new template for user source routine source_newgen.f which is distributed since the 4.1.0 release and it is announced among its technical improvements (Release of FLUKA 4-1.0):
TECHNICAL IMPROVEMENTS:
An additional version of the source routine, source_newgen.f, with a cleaner structure that facilitates access to newcomers and several built-in options, has been made available as beta release in the src/user/ directory. (The traditional source.f is still fully operational and preexisting user-customized source routines remain totally compatible).
You can find info on how to use it in the dedicated lecture: