What grammatical construction or use of a particular word do you have that you want to know is correct and proper?
I find myself often winging it with "themself/themselves" and it seems to be like themselves is always colloquially correct when there are multiple preceding nouns you're referring to...
Otherwise if there's only one antecedent or whatever, its themself
I've seen a few people prescribing "themselves" even for the singular, and interestingly enough Wiktionary mentions that "themself" is sometimes proscribed. Based on that I'd argue that the linguistic community didn't "settle down" on the rules of when to use one or another.
That said, personally, I use it like you do: -self in the singular, -selves in the plural. Same deal with [our|your]+[self|selves].
Weird - I never noticed that the third person reflexive forms typically use the main case (him/them) as a basis, while the others use the possessive (my/thy/your/our). No idea on why this difference.
That said "hisself", "theirself" and "theirselves" don't sound bad to my ears.
It also doesn't help that the third person feminine is ambiguous. There's often no distinction between the accusative "her" and the possessive "her" (except when the pronoun appears in a different part of the sentence and becomes "hers" - fuck I hate English), so it could be interpreted as fitting either rule.