Edit:
Bah: I was sleepy and forgot things:
Mod List:
multimob-1.0.5.jar
primitivemobs-1.2.3a.jar
Forge 14.23.5.2810

So my Multimob config has 1 custom entry: _EndZombie#minecraft:zombie where the only default setting I changed was to swap its spawnDimension from 0 to 1.

https://gist.github.com/TeetoKaziim/f67840312230822bb1c583c1ae93f64f

The result was truly overwhelming zombie spawns in the End:

215 entities in normal difficulty

Shoots down to 0 in Peaceful mode.

Also: It seems like the "spawnlimit" setting isn't doing anything: Setting the spawnlimit to 10 and reloading the game, and flying far enough away to despawn everything, I end up with a lot more than 10 zombies in very short order:

I've been testing with the spawn limit settings for the past few days and I'm as stumped as you. I'd really appreciate some help on the subject! I believe the spawn limit is normally supposed to be around 2 or 3 max for a lot of mobs, and even then I still get about double that amount over the course of a minute.