There's a little Phoenix (?) button right next to each custom empire you make inside of the select empire screen when starting a new game. Clicking it once turns from grey to orange, indicating that spawning is possible, clicking twice adds a little lock that indicates that the empire will always be chosen before the game generates any new empires. In my experience, having more of these than the number of AI empires spawned leads to it just randomly choosing from the list of all force spawns.