Possible to play with all 12 major Races?

No, it's not possible. I tried a mod to simulate the experience, though, described here: https://forums.galciv2.com/397714

The Yor didn't colonize, were horribly weak, and eventually were conquered. It worked for the Thalans, however, who acted pretty much like an eleventh major race, albeit a somewhat small one.

I wish we could get past this arbitrary headcount limit. I know the game will run slower with 12 or even 16 opponents (if I put my custom races in too), but it would be worth it on an Immense map.