Hi Sascha,
For Pythia 6:
Take this tar file file: https://root.cern.ch/download/pythia6.tar.gz and replace the pythia6416.f with the version you prefer (I just tried with pythia-6.4.28.f), and then run the makePythia6 script which corresponds to your architecture
For Pythia 8: you can use any version of Pythia 8 with ROOT 6, but with ROOT 5, you have to use Pythia 8.1 at most
If you don’t manage to get it working, please post the error message you get
Cheers, Bertrand.