AFAIK the NA units always start in the same regions, ie there are no variables which might vary starting locations.
As long as an NA unit is neutral it basically has not affect on the game, will not and cannot be attacked. Only RGD's can affect them. The AI can use RGD's to bribe NA unit, but doesn't do it all that much from my experience.
Since there is only one NA faction, when the Great Sioux Uprising event fires ALL neutral NA units will become active as an enemy of the Union until the event closes.