As for 1932, if Zentrum wont coalition with you then having Otto Braun as an advisor can help. He has "Negotiating with the Coalition" which improves relations with them. It also helps not to try to associate with the KPD, though you can do both. Obviously there is the Inter-Party Relationships card, but that's more useful for the KPD than anything else. I usually don't have much of a problem with them even when I'm passing hella welfare and raising taxes out the wazoo.