I have noticed that none of your mob are affected by those mods, probably because they are not considered hostile?

If the purpose is making them not despawn in peaceful mode. What if you try to do it in a way like Cataclysm or Mowzie's Mobs?

(These two mods mobs don't despawn in peaceful but also work with mods that affect behavior)

no no no
Do you know Champions mod? It lets you add "stuff" to hostiles. Eeeabs mobs do not work with that

Some mods change hostiles behavior, your mobs don't work

Can't add potion effects from other mods? Cataclysm mod Stun effect, Mowzie's Mobs Frozen effect?

Are only some mobs incompatible? Can you give me an example of that entity?

Every mob is.

One question, how did you make your mobs not despawn in peaceful difficulty?

I override the Mob.shouldDespawnInPeaceful() method to set the value to true

Alright I'm unsure why your mobs don't seem to be "monsters"

I believe Champions mod only works for monsters

Another example is Guard Villagers mod, none of your mobs are considered "monsters" by them

Uh, you mean creatures don't disappear in peace mode? Not sure what you're trying to say...

I have no idea at the moment, I have to look at the Champions mod source code, thank you for your feedback, I will add this issue to the mod development schedule :)

whatever you did it worked! (latest commit)

Really?? I was surprised...

Yeah it works great. The mod seems to work the same but compatible with others now.

The only change may be that immortal skeletons despawn in peaceful (the other immortal don't)

OK,I will correct the point you mentioned, at present these immortal mobs are unfinished :)