where to add new government types?

They are in TechTree.xml

You can add new techs, but I believe ShadowTek investicated the governments themselves and found they are limited to the existing number (he worked around it by combining governments with ethics).

yes, what kryo said is the case. the game will only support a player having 4 gov't options. however, if one or more of those options comes from an ethically dependant tech, you can mod the game to contain more than 4 gov'ts.

here's his mod forum: https://forums.galciv2.com/?ForumID=348&AID=119845#927936

the techs for the govts are in techtree.xml, but the bonuses provided by govts are in english.str (unless you're playing a different language configuration).

OK well that's what i was wanting to do, add a unique government to go along with ethical alignment.

OK, read the posts in the link, but i'm a little confused... Where do i add the actual DATA? what file? Can't just be techs.xml

PS: this is news to me, but the gov'ts are basically hard-coded. you can write new techs to access them, but not actually change their procution bonuses or sensitivity to morale. the lines in english.str affect the gov'ts display. so you probably notice how the govt techs give morale and influence bonuses: that you can change. you'll get those bonuses regardless of whether or not you use the govt. the production bonuses, however, are hard-coded.

what shadowtek has done is this basically this. his mod makes the techs that access the 3 advanced govt's ethically-dependant. so there are 3 techs that will give you "republic", 1 for each alignment. the bonuses provided by the tech are unique, but the govts are still the same.