I'd say there are expectations, rather than rules. If you know your audience is likely to care about these sort of things then it's indeed a good idea to correc

I'd say there are expectations, rather than rules. If you know your audience is likely to care about these sort of things then it's indeed a good idea to correct them, and it's usually best to be conservative in formal writing. But that doesn't really mean there are "rules" which cover any and all usage of the English language. Joining words to one compound word is pretty much the example of language evolution.