The easiest way to create new AI behavior would be to edit the AIStrategyDefs.xml and have a custom Personality Type for them. Problem is, even as they can be defined within their own file, the XSD schema disallows custom PersonalityTypes.
When I created the Favixi race for the Faction Contest, I set the AI behavior to be only for minors and with an Expansion trait. The trait was not used by any official minor race so it was one workaround I found.
