It depends on how much effort you put into changing SDK to make different schemas work. There are some files where that may not be worth it, like schemas. But for casual modders that just want to add new units and buildings, I don't think we have to worry about different schemas yet.
It took some time, mostly to relearn how to expand dynamic arrays, but I finally got the mod to work. It reads in extra files without putting original Firaxis files inside the mod's folder.
Right now it's just for unit infos. What it does is read in files that match *_CIV4UnitInfos.xml and add those entries into the game. So your folder, C:\civ4\Mods\MyMod\Assets\XML\Units\, could just contain MyMod_CIV4UnitInfos.xml and nothing else. You don't need the original files. Whereas before, you would have to add your new units into the huge CIV4UnitInfos.xml and put that inside your mod folder.
If you don't want to use the vanilla units, then you would make your own CIV4UnitInfos.xml and put that into your mod folder. This works just like before.
If you want to borrow another mod's unit infos, copy that file into the units folder and rename it, like HisMod_CIV4UnitInfos.xml. So your units folder would have two files, MyMod_CIV4UnitInfos.xml and HisMod_CIV4UnitInfos.xml, and both will be added to the game. Or you could just use his file and not even make your own.
Forgot to mention, you still need a complete CIV4UnitClassInfos.xml for now, that also contains new entries for new units. So I still need to work on that and the art defines.