Battle of the Gods Scenario (Allow Other Races?)

Scenarios have a fixed starting race. If you want to play as another race, you would need to create a copy of the scenario with that race set as starting race.

Also, the scenarios were created for DL and haven't been updated for DA and, more importantly, TotA, so there will be bugs in them.

How do you do that (make copy of scenario and set new starting race)?

You have to use the Scenario Editor for this. The 'GC2ScenarioEditor.exe' is located under 'Galactic Civilizations II - Ultimate Edition\Twilight\Tools'. There are faster ways to get there (like the game launcher), but I don't know if they are available with the Steam-version.

In any case, you need to load the file 'Battle of the Gods.gc2scenario'. You can find it under ' Galactic Civilizations II - Ultimate Edition\Data\English\Scenarios'. Now, change the 'Player Race' to the one you want to play. You should also change the 'Scenario Title' slightly. Otherwise, you might confuse it with the original. Once that is done, save it via the 'Save as' option and give it a different name.

Still, this doesn't fix the bugs. Primarily, that you don't start with all the techs, because they didn't exist in DL, and that the Thalan are not available as an enemy. To fix the latter, go to the 'Teams' tab, and click on the box in front of the 'Thalan' entry to enable them. To fix the former, go to the 'Technologies' tab and click on 'Terrans'. Then, click on the first tech under 'Available Techs', scroll down to the bottom, and shift+click the last tech (this should select all the techs). Now, click on the 'Add' button in front of the 'Starting Techs' box, and then on 'OK'. Once that is done, do the same for all the other races.

Thank you.  I got it working.  Part of the trick is to get rid of the soil enhancement line of techs to make sure that everyone starts with a capital city module AND a starport.  If you don't get rid of those techs, the bug that can happen is that your capital city module and initial starport start on an unusable tile, meaning that you might begin with 0 population and no starport.  

Still playing the Torians.  I was considering the Thalans for their massive manufacturing output but... Torians are better.  Super breeder and Central Mines means EVERY colony you establish can produce more colony ships.  No bottlenecks.  The Thalans are bottle-necked by population.  

Also, the Torians start out as "good," which means you can commit lots of evil acts to make yourself neutral.  

Is there a way to make the AI build capital ships (if you give them all the techs) instead of those dinky little fighters they like so much? 

If the AI has the tech, it will do so eventually. However, you can't force this.