These ground rules seem reasonable.

In general when people say "I want to tell you something, but you have to promise not to get angry/offended/etc.", my response is along the lines of:

"I can't and won't promise that. I do promise that I will make an effort to temper any knee-jerk reaction I might have, and to give thought to your words and to my response before I say anything. I try to do this in all of my interactions with people whom I respect, but in this case I promise to make a special effort."

And if that's not good enough... well, then it seems my interlocutor doesn't care that much about telling me whatever it is they wish to tell me.