Well, right now it's not possible to have specific units be different just because they're from a different nation and everything.
The Civil War thing might be possible using lua code, but I don't really know much about that. I forgot everything I ever knew about lua, since I don't work with it anymore.
For the units, you need to
- make the unit. This is done in the units.ruleset file of your ruleset, and it's pretty easy to understand there.
- add the graphics. For this, you can basically copy the /data/amplio2.tilespec file(or the tilespec of the tileset you want to use, default is amplio2), but add a new file to the long list of included files(lines 66-96 or something). This new file would be pretty much like /data/amplio2/units.spec, and as the tags, you would use the ones for your new units. You also use that graphics tag in the units.ruleset file(e.g. "u.spacemarines").
Two wiki pages you could check out are Editing Tilesets
and Editing Rulesets
. I'm not sure if they're up to date, but most of them should be.