I like to really keep it in check. I want a FEW of each. If I don't play a vampire sim, I only allow the premade vamps in my game. I delete aliens so there' only like 2-3 in the game (if the AI actually switched on their human thing on by default... But unfortunately the aliens generated for Alien Night at the bars don't have, or don't use, disguises. So after a while you have 10 or so permanently green / blue aliens everywhere). I uninstalled Supernatural for Sims 3 because when about 50% of the population is not-normal then it's WAY too much for me.