The custom race saves are xml also. You can edit them to set the value of things that aren't even available with customization (like miniturization). There is a section for techs too, but you'll need to find the proper names for the techs you want. It's similar to the raceconfig, but saved individually for each custom race (including custoized ingame races). They are the .raceconfig files found in MyGames\GalCiv2
The AI uses the raceconfig.xml for it's stats. Editing the raceconfig.xml will allow you to change the bonuses for the AI, while editing the custom .raceconfig files only affects the player. This means you can give the AI bonuses when they are an enemy, but have them be the normal config when you play as that race. It could work the other way too, but do you really need to make the game easier than it is?
*Backup any files in the main game directory before you edit them*