The frienship Polonia with Cracovia was based on tradition, it's the oldest friendship I think. For example Lech Poznan would like very much tu build a friendship with Widzew Lodz, because they have very good hooligans and Widzew one year ago have build friendship with Ruch Chorzow because they are also good in hooligans (called Psycho-fans). You know, many of supporters has different way to build the friendship.
The hardest hooligans friendships are:
Arka-Lech-Cracovia
Wisla-Lechia-Slask-Jagiellonia-Motor
Legia-Pogon-Zaglebie
Widzew-Ruch Chorzow- ??? (I don't know)
The friendship based on tradition:
Polonia-Cracovia, Polonia-Bron, Polonia-Arka (???)
and I know only this

Mayby someone will tell much better then I. I'm not so good in English (better in German)
Greetings