does it affect unit types? - Some units are set by culture, but you can change this in the entry for that unit in export_descr_buildings.txt
does it affect ancillaries? - Sorry I don't really know
does it affect allowable building types? - Same as units, I think there may be some culture specific buildings, but this can all be changed in export_descr_buildings.txt

About the Romans, apparently the romans do eventually declare war on each other, though I've never seen it unless I started it..

I haven't tried setting them at war to start with. I guess those at war with the senate would be outlawed, never to be accepted again, but the only way to be sure is to test it...

sorry about the time it took to reply