For example almost everyone agrees that single letters are pluralized with an apostrophe. It is somewhat less common to prescribe an apostrophe in the case of a number either. When this is not allowed, an exception is sometimes made for pluralizing the number zero so that it is not mistaken for the letter O.
It is fairly common to pluralize period-less abbreviations with apostrophes. The New York Times, for example, follows that standard. It is more common still for abbreviations with periods to take an apostrophe.
So basically, no. You don't get to just say "this is the rule for apostrophes in all cases" because that's not how the written word works. We (or the publication we are writing for) can choose our own editorial style, and screw anyone who doesn't like it (although you should be consistent, of course).
But interestingly enough, this is one situation where there can be a universally correct answer: a trademark. If Yahoo! puts out guidelines for how they would like their trademark pluralized, and they say it should be "Yahoo!s", then that is what everyone should use. If they say it should be "Yahoo!'s", then that's what everyone should use, even if it violates their usual rules.