Clair, it’s possible to exclude entire phrases out of an otherwise “catch-all” GREP query, but you have to list each one of them.
You can try this:
(?!(Assistant|Chief))(\u\w+ \u\w+)
This ‘looks ahead’ for either “Assistant” or “Chief”, followed by a word break — the break is to prevent the Chieftains Museum, Rome, Georgia, being ignored. (A somewhat constructed example I admit, but you SHOULD check for entire words, just in case.)
I THINK you only need to check the first word here; in the case of your “Assistant Chief”, the “Assistant” is the one that gets ignored right away and ID will not contemplate “Chief”, unless it is followed by yet another capitalized word. And then “Chief” and that word in turn are ignored, unless there is a FOURTH capitalized word following (in which case only the two last words are found, as presumably is the intention here).