I'm at work atm and i dont know when i'll be able to test this, but in the descr_strat.txt each faction is described as followed :
faction russia, trader smith
ai_label default
The ai_label has 3 possible values : catholic, papal_states, default.
From my campaigns, i've realised that making peace with "default" factions (russian/byzance and the muslim nations) was much easier. In fact as a catholic nation it was much easier to stay at peace with them than with my neighbours back home even after sacking quite a lot of their towns.
Did anyone tried to change the "catholic" ai_label to "default" to the catholic factions ? (i'm at work atm :-/)