I've been dealing with some interesting interactions in my mob farm as of late, and I figure this is a decent place to dump some findings.
1. Blaze and Blizz are enemies and attack each other on sight. This is cool and all, but if a Blizz misses its target with an attack and the attack lands on a horizontal surface, it covers that block in a layer of snow. Similarly, a Blaze shot will light a horizontal surface on fire. Both of these things work to reduce the efficiency of my farm, and the fire can actually destroy cursed earth.
If a bystanding mobs are caught with a stray shot, things escalate quickly. I had just finished a full renovation after an unexplained meltdown, and upon restart, I witnessed a fight between two Blaze and a Blizz. One of the Blaze missed, striking instead a witch. The witch retaliated with a splash potion (I'm assuming poison), which pissed off a bunch of other mobs. The MFR grinders kicked in before any more damage was done, but all it would take is a creeper getting tagged and all bets are off.
2. Enderminies and Wither Cats hate EnderZoo variant Creepers. This isn't so bad, as these Creepers don't grief the farm, but its worth a note.
3. If the walls of your mob farm are thin (1-2 blocks), a Wither Cat spawning with a Wither Witch is able to spawn outside the walls (due to the spawn mechanics at play). This is harmless, for the most part, but its annoying to have my maintenance hatches fill up with cats.
4. Six Witches contained over a Well of Suffering allows for nearly infinite LP, but placing them into containment using safari nets is bound to aggro at least one of them. That one bitch will throw a splash potion which is bound to affect at least one other Witch. Then, stand back, pump techno, and watch the Witch rave party, as they cycle through heal self, heal self, splash potion.