Not sure if this is a bug, or if I've lost the plot. I've tried various rules along the lines of "mob:zombiePigman", "mob:EntityZombiePigman", "mob:minecraft.zombie_pigman", but none of these have worked. Every time I get an error that states "can not find this mob". So, how do I go about setting a rule related to zombie pigmen?

Never mind, I just say the wiki; "PigZombie". It'd be great if the name used by the mod actually matched the vanilla minecraft entity name.

Let me check.

According to the code,
register(57, "zombie_pigman", EntityPigZombie.class, "PigZombie");
Seems entities got new registry names since 1.11. I will add the support for those new names in next version.

Thanks for reporting.