Various non-standard and incorrect plurals are currently in use

I noticed that the rules for Pluralize currently result in many words being pluralised as something that is either wrong or highly non-standard. The list of words I'm proposing changing is:

octopus, hippopotamus, premium, stadium, podium, ultimatum, condominium, euphonium, harmonium, criterium, spandex, himself, herself, itself, myself, oneself, all the "-ium" elements (e.g. aluminium, caesium).

The main one I first noticed was "octopus", which is currently pluralised as "octopi". That's...not wrong, per se, but it is the third-best option, and only an acceptable option at all because enough people have hypercorrected based on the mistaken belief it is Latin that it's become correct through use. To quote Wikipedia:

An example of a common hypercorrection based on application of the rules of a second (i.e. new, foreign) language is the use of octopi for the plural of octopus in English; this is based on the faulty assumption that octopus is a second declension word of Latin origin when in fact it is third declension and comes from Greek.[

A better etymologically-correct option is "octopodes" (pronounced oc-top-uh-dees), but the most widely-accepted English plural of octopus is to treat it like a modern English word, and go with "octopuses".

The other words I propose amending mostly follow that same logic. The most widely-accepted English plural seems to be to treat it as a standard English word and add an "s" or "es". Most are switching an "-ium" from "-a" to "-iums". On other "-us" to "-uses" rather than "-i", and one "-dex" that should be "-dexes", not "-dices" (though there's also an argument, according to Wiktionary, for it being made uncountable instead).

Most elemental "-ium" names can be either countable or uncountable. When countable, the plural is always "-s". But uncountable seems to be more common, so I propose making them uncountable.

The last set I was looking at were reflexive pronouns. English only has one standard third-person plural reflexive pronoun: themselves. "himself".Pluralize() would, in this model, return "themselves". However this would break the unstated assumption that x.Pluralize().Singularize() should always return x. If that is considered a problem, then then reflexive pronouns should be set as uncountable, so "himself" -> "himself", rather than the highly non-standard "himselves".

Depending on how you feel about round tripping, Singularize could be set to accept all variants of plurals. So "octopi".Singularize(), "octopodes".Singularize(), and "octopuses".Singularize() all return "octopus", even as "octopus".Pluralize() always returns "octopuses" (etc. for other words).

I also have a list of a bunch of other words I considered but ultimately decided not to propose amending. Because although my own usage is "-s", I found evidence that both "-s" and "-a" are in significant general usage; or because the word was obscure enough that I couldn't find good evidence either way; or because the "-a" usage, though rare, is common in specific academic/professional spheres. These include:

aquarium, auditorium, gymnasium, planetarium, geranium, nasturtium, factotum, pandemonium, sanctum, delirium, rectum

I have the code change for this basically ready to go, subject to any specific changes, if you want it.