For following you, you can set up script boxes that add and take away path nodes, so when your player collides with the script boxes one by one, the monster is forced to keep moving with your character. It's not too intense as far as scripting is concerned but it would take a bit of testing and playing around to get working correctly. :3
To make him not attack the player, just put
SetEnemyDisableTriggers(string& asName, bool abX);
into your script when your monster is spawned/activated. He won't attack or react to the player. He's perfectly safe.
But I think the music still plays, so you might have to make a copy of him and edit your copied monster with the modeleditor so you can remove his music.
EDIT: For attacking another monster... that's pretty tricky. There's not much you can do because the script for making the monster do a certain animation has never worked. However, if you play around a bit, maybe just have the friendly monster (which would be your edited monster) run really fast up to the enemy monster, and set up a script box, so that when your friendly monster gets to a certain point, the enemy monster is deactivated, and with clever useage of PropForce, you can activate a ragdoll of the enemy monster and it looks like friendly monster runs into/attacks the enemy monster and kills him.
Just throwing out a suggestion. It sounds really extensive, but it would be pretty simple, like I said, you just have to do some playing around and testing to see what you like best.