Here's how I define the population of a planet. Each "Proportion" number represents how much of the whole the character takes up. So an "EnemyBlob2" with an 8 is 8 times more likely to appear than an "ItemRevealCave" with a 1.

So at runtime I just add up all the numbers, pick a random in that range, and grab whatever lives in that part of the range. #GameDevelopment