There is a need for rules, a code, something to let us know what boundaries the care-takers have in place. Simple rules are the easiest to understand. Too much text and it easy to stop paying attention or someone decides to argue rule 413, paragraph 3, sub-clause 5.8. Perhaps a summary or preamble can be kept in the foreground and a more detailed version can lie a click away.
"Be nice" is about as simple as it can get. It is subjective, but the idea is plain. Moderators must be willing to set examples and clearly state when and why the line has been crossed. An even handed approach is important to provide room for new-comers who are just learning, and room for those who get carried away to apologise and keep contributing to the group.
Steve, as the Ultimate Care Taker, you are always going to have the "final word", so the rules must represent what you think to be right and fair. You are doing a good job. You have a healthy online community here.