TK has added nation-specific number decals for several WP states, that's why there are now more silver skins for the MiGs in SF2.
The nationname in the Texture.ini is actually not that important, since an aircraft's nationality is actually determined by the Nationname used in the aircraft_data.
See this stock SF2 F-4F? The nationname given in the Texture set is still WGermany, but I had changed the nationname in the data.ini to USAF.
It has now American insignia, but does also use the German decal numbers...which makes sense, since that's how the decal.ini set up.
What I'm trying to say is this, you can use the same skin for several nations, but it won't make any difference in a random single mission, since AFAIK an aircraft can't have more than one nationality (Unless NATIONNAME has the same flexibility as ATTACHMENTTYPE).
If you want two different nations flying the same aircraft, then you have to go a bit further and create nation-specific copies of that plane.