Yep, and AFAIK... each of these (with unique names) can be used as opponents straight out of the DA folder once you have edited whatever you need off their individual xml files.
The multiple configs is a bit more tricky - indirectly. You'd need a sort of tracking process to prevent coincidental (even accidental) loadings of two copies of the same custom race since that would create a conflict. But as long as you rename a few call-lines (Like: Terran-A & Terran-B, etc), i don't see any obstacles to have any number of 'same race' files ready for action in the default folder mentioned above.
In fact, that's exactly how my X-Worlds Mod works. It uses multiple (36 of 'hem available any time!) custom races instead of bothering with total conversion (with a maximum of 12 plus the usual
custom slot) editing through the core raceconfig.xml. The advantage being that
any game setups still can have access to the default races simultaneously with the modded addons. Nothing i like more than to pit X-Com and/or Sectoids against Yor & Korx (etc) once in awhile!
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