A general rule is to be nice to everyone. That is just.. normal.
Another rule is that as long as you do not offened other people, by spamming, offensive language, anything that offends the opposing person/people, then it's okay to do as you wish to.
Abuse of it would be limited to the troublemakers. If someone says "Hi Jose" to someone, and Jose then reports that person for it, just because "I didn't like it", and you know it's to cause trouble, then you can, if you haven't alredy done it, put up the rule saying "Do not attempt to abuse the rules", and there you have it.
People should be allowed to have fun, that is what gaming is about, but letting members of your server go out and harass another member with the intent of provoking/annoying/making fun of then that should not be accepted.
Many can be mature, yes, but you also have those who think they rule in everything, and must correct everything other people do, and then team up with friends to support their claims/reasons/ideas/meanings, and that is not being mature.
One of the most important rules in a game are the Player Relations rules. You cannot have a server or forum without them. A few neglect it, or simply forget it, but it is what makes the community good.
Imagine if every server was filled with people like Zeel, or that most of the people who have played on a server are rude etc. It's not a nice thing to have for your server. And if the Gm doesn't care, phew then it's bound to go downwards.