For my money, witches and heretics are a distraction from the basic game. There have been several suggestions on how to mod them out-- but NOTHING works AFAIK. That includes rebel spawn rate AND the descr_campaign_db.xml file data. The latter DOES seem to work with inquisitors, but the number of heretics & witches seem unaffected by any changes.

Realize they can be good for training up Priests-- but my experience is that by the time you get to the middle-game or so, and are pretty much "number 1", you aren't going to get any more Cardinals no matter what you do.