Q:

What are the other unwritten rules?

I know about the code against fighting on holy ground and multiple immortals ganging up on one- are there other unspoken laws immortals follow?

A:

Per the wiki, there seems to be a number of "rules" that immortals are supposed to follow. That being said, over the course of the three two cinematic films, five made-for-TV films and TV series they break pretty much all of these rules, usually with only minimal consequences. I think it's best to say that these are more worldbuilding guidelines or general policies than hard and fast rules by which they live and die:

There Can Be Only One
You have to play The Game (e.g. you have to locate and fight with other immortals)
You will feel the call of the Gathering and go to that location.
No fighting on holy ground
Duels should be one-on-one, as should quickenings
Minimise contact with outsiders (don't go public)
"Dead" immortals (those seen to be killed) should break ties with their former communities
Most wounds will heal post-immortality, some will not.
New immortals are afforded a grace period in which to learn how to fight and to be trained in the rules above.

