"Follow the roleplay of your ID" is all that is needed. It is best to avoid any unnecessary rules fluff. Telling people they can't ally with hostiles just brings up more questions than it answers, since it is not clearly defined anywhere who a hostile would be for each individual ID, and this diplomacy may also be different for various subgroups of an ID.
"Apply common sense" is really quite underrated. I think most people are either capable of doing so, or will either ignore the rules or find loopholes no matter what the exact ruling is.