I haven't had any success (CURSED MINIDUMPS! I wish IC would release a tool to decipher the stupid things!) modding that file, but others have.
It's not a very complex file structure, thankfully. If you're adding a new race and you need to set up its starting units/structures, here's what you need to do:
1) Add the race into the file:
The designName is what you will be using to refer to your race within the GalaxyScenarioDef file ONLY. The entityDefName is the name of your race's entity file. So just add yours and, as always, increase the counter.
2) Add the ships and structures you want it to start with to the list of available ships/structures:
Same thing as before. designName is internal to the file, entityDefName is the name of your ships/building entity files. You only need to add the entities you need as part of the starting conditions, you don't need to add every single one. And of course, increase the counter.
3) Put 1) and 2) together in a home planet template for your race:
Just copy the entire 'group' block to add another group. The param field is your race (use the designName you gave it), and the item fields are your structures and units. Update the counters appropriately. This template is for normal start, you'll need to add one for quick start also:
Same idea. So you're adding 2 groups for 1 race and so you should have a total of 8 groups under the "Template:DefaultHomePlanetSetup".