The problem is making me crazy, but when I set level-type to RTG, custom biomes from Mo's creatures become rarely, then I realized that BiomeTweaker overrides not only biome generation, but mob spawn also. This is shit, cuz I need to write separate file for every f**ing biome(147 total) to set up mob spawn rates, because standard weight for Zombie is 100 and Min Group Count is 4, but standard weight for mo's mobs is 4-10 and min group count is 1-2, so there's only zombies, creepers, spiders and rare rare cave orc. In Custom mob spawner I can pass spawn rates for all Overworld biomes.

Okay.... BiomeTweaker doesn't override anything unless you tell it to... I'm not really sure what your issue is.

Does forAllBiomes() not do what you want...?

Closing this for inactivity.