You cannot spawn other hostile mobs in a custom biome except than the casual ones (ex. Jockeys/giants/nether skeletons/ strays / blazes/ etc...)
only ones that seem to work are:
[{"mob": "Spider", "weight": 10, "min": 4, "max": 4}, {"mob": "Zombie", "weight": 10, "min": 4, "max": 4},
{"mob": "Skeleton", "weight": 10, "min": 4, "max": 4}, {"mob": "Creeper", "weight": 10, "min": 4, "max": 4},
{"mob": "Slime", "weight": 10, "min": 4, "max": 4}, {"mob": "Enderman", "weight": 1, "min": 1, "max": 4}]|

I have tried the correct IDs and Data values, they dont work
And i consider this an issue and not a feature request because this pic that you have at terrain control's front page says otherwise:
http://i.imgur.com/ZKIHP.png

(version 2.8.2 Snapshot)

Hi there,
If made my own Hell Biome and that worked for me.
SpawnMonsters: [{"mob": "Ghast", "weight": 50, "min": 4, "max": 4}, {"mob": "PigZombie", "weight": 100, "min": 4, "max": 4}, {"mob": "LavaSlime", "weight": 2, "min": 4, "max": 4}, {"mob": "Enderman", "weight": 1, "min": 4, "max": 4}]

Here is the complete Biome bc file:
https://github.com/BakermanLP/ProjectRET-AoT/blob/master/templateTC/WorldBiomes/Hell.bc

And also try to use some of the later builds out of the jenkins:
http://build.mctcp.com/job/TerrainControl%20-%20Master%20-%20Gradle/default/

I personaly use the 162 build.

Michael (aka BakermanLP)