Re: Trip, Disarm, etc.
Zombies are mindless. Tactics are beyond them.
Bearing that in mind, certain actions make sense for zombies to perform, others do not.
Zombies should grapple. Classic zombie stuff. They grab you and munch your succulent, living flesh.
Zombies should not disarm. Unless they somehow disarm without even trying to, like how an adhesive mimic does, a zombie choosing to disarm an opponent requires some thought to take place. Zombies arn't big on thought.
Likewise zombies probably should not bull rush, overrun, or trip. Those are all tactical decisions; they are options you choose to do other than grabbing the living thing and eatting its brains.
In the end, it all boils down to the Awesome Test, as has been mentioned.
Question: Is this proposal super awesome?
Answer: If yes, then do it. Rules be damned.