Latest build 3.0.6.245 no longer has configurable entity IDs.
Details:
* The entity ID configs (for Evolved Spider, Tier 1 rocket, etc) in the config files now have no purpose and can be deleted or ignored. A newly generated config file will not have these settings
* All IDs are internally generated, in a way which should make ID conflicts impossible
* This is intended to allow compatibility between Galacticraft and Orespawn, especially
Points to watch:
* An insanely large mod list could in theory "run out" of IDs for mobs, giving a crash with the error message "No more entity indicies left". If that happens Galacticraft or some other mod would most likely have crashed with an entity ID conflict anyhow. Minecraft just cannot handle more than 255 different types of mobs in the game, it's a hard-coded limitation. For an idea how many mobs you have in your game, look at the spawn eggs in NEI. Most modded games should have 20 or 30 eggs there. I've never seen more than 100.
* If you want to push the limits, use several mods which all add a large variety of mobs. How about Galacticraft, Orespawn, Mo' Creatures, and Grimoire of Gaia.
* If you're in Creative mode, please check that all the different types of Galacticraft evolved mob spawn eggs still work OK. They do work in my testing.