I'm also using mo' creatures and have heaps of centaurs spawning. One more thing is that I have tried reducing the spawn rates for all grimoire mobs to be 1/10th of their original value but still see a lot spawning. Is this because of mo'creatures or something? I see heaps of ant packs of 3-4 in the desert when the value is set to 2 in the config. Mo creatures uses a frequency integer that is between 1-12 usually so reducing grimoires mob spawn chance to lower numbers might still result in lots?
Ya so I mentioned this in my last post but I'll elaborate a bit further. It looks like you and I are in the same boat, so we can probably safely conclude Mo' Creatures is the cause. I tested this pretty thoroughly over the course of 4 or so hours by running both mods and setting all of Grimoire's spawn rates to 0 and then flying around for a few in-game days, observing the spawn rates of Grimoire mobs, adjusting them in the configuration, and then starting a new world to make sure the effect takes place and then repeating the process over again.
Basically the conclusion I came to was that Mo' Creatures custom mob spawner completely breaks the spawning for certain Grimoire mobs, but not all of them. It's pretty easy to tell which ones by setting all of the Grimoire spawn rates to 0 and then just waiting to see if that mob still spawns. Like you said, Centaurs and the Ant mobs were the worst so I focused on them. Not only do they ignore the spawn rate being set to 0, but I'm pretty sure there's no change between 0 and 100 with those two mobs in particular. Their spawn rates at 0 with Mo' Creatures installed seemed identical to their spawn rates at 100 without it installed. As in, somehow Mo' Creatures is forcing them to spawn at their max rate and it's overriding Grimoire's configuration. It's worth noting that the Anubis mob was almost as bad as those two in deserts at night. Just like them it ignored the 0 spawn rate and seemed to spawn at 100 no matter what it was set to. On the other hand I did notice some other Grimoire mobs still spawning with their spawn set to 0, but they weren't nearly as bad as those three and most mobs seemed to function correctly. In addition after setting all of Grimoire's spawns to 100 and Mo' Creatures to 0, I absolutely couldn't get some Grimoire mobs to spawn, no matter how long I waited or how much I adjusted spawn rates. This leads me to believe it's actually fully preventing certain mobs from spawning as well.
In my opinion the Ants and Anubis aren't a big deal since deserts are so empty anyways that they're not really preventing anything else from spawning. The Centaurs on the other hand are a problem. I won't be able to try the custom jar ATotalGit made for the Centaurs until later tonight but it seems that would solve that issue so I'll test it when I can and post the results. I think the big issue here though is that Mo' Creatures seems to be preventing certain mobs from spawning at all so semi compatibility is probably the best we can hope for. Personally I don't want to miss any Grimoire mobs so I'm gonna pass on Mo' Creatures once I'm done testing this. But at least there's some compatibility there between the two. If you think this is bad just try running Mo' Creatures with any other mod that either alters biomes, adds new biomes, or adds new dimensions and watch the incompatibility train wreck that causes.