It looks like that you can modify all the spawning friendly creatures in a Custom Biome (ex. you can add Polar Bears in desert biomes with high temperature etc)
But you cannot modify the spawning of the monsters. If you want Strays Skeletons for example, you will have to use ReplaceToBiome: Ice Plains for example
But if you do not want some of the monsters of that replaced biome (ex. you want to remove the creepers, or all the monsters) terrain control wont let you, and it will just add every monster mob that exists in the parent replace biome in the values and weight it has (default).
Is it possible to fix this bug? If yes i would love to see a new updated version