I'm not sure if this is a bug, but I've managed to get all 10 major races and the custom race in one game. When setting up the game you have to be the custom race, and then when selecting the number of other races set it to 'Randomize'. One of the races should be darkened - click on it and then select Enable. Set the difficulty to whatever you want and then start the game with all 10 races at once (plus your own).
I'm pretty sure this is a bug (or at least, unintended) because in the places where it shows the stats of the races (1st, 2nd, 3rd etc) it has "String not found" wherever it should say 11th

Has anyone else done this before?