The rule I mentioned is for Abbriviations/Initialism, where individual letter is pronounced seperately, thus effectively has more than one syllable.
The ones which you ve stated now, AIDS, NATO, UNESCO, RADAR, LASER etc are Acronyms (a word formed from the initial letters or groups of letters of words in a set phrase or series of words). An acronym is when the capital letters are pronounced as a word and not individual words.
Acronym and initialism - Wikipedia, the free encyclopedia initialism vs. abbreviation vs. acronym
I cant think of any stress rule for them, because all of them are monosyllabic (one syllable).