Basically, I edited the "Custom Maps" portion of this: https://www.galciv2.com/mods/gc2modsdoc.htm  Iwrote it first in notepad, then saved it as a GC2MAP file, along with another copy as an XML for editing.

Anyway, I did try it there, and that's the instance described above where I did manage to get it working, but it only loaded my race and none of the opponents, despite four opponents being selected.  Not to mention it renamed my star Sol (I was playing as Terrans), the planet Earth, and the other planets (which were supposed to be inhabited by the other races that were never loaded), Sol II, III, etc., despite all of them having CustomName activated.  There were also some anomalies in places where I didn't specify, and that modding guide says they should appear unless you specifically place them, unless that's one of the differences between 1.2 and 1.4.

A similar thing happened when I tried playing as the Arceans.

Through 12?  This is Dread Lords, it only goes 0-11 if you count Custom and Dreadlords.  Do those also need homeworlds, or is it just the ten (in this version) Major races?  I added five more homeworlds (the original map was only intended for five players, but now I have ten homewrolds in total), and it still only loads my race and does all the other things described above.