I have a couple of English usage manuals on my desk (Fowler 2e and Garner). Fowler says it's silly to restrict etc. to things rather than people, while Garner says to use etc. only for things, et al. only for people.
They seem to agree implicitly that for an actual Latin speaker, this distinction would make sense. What is the distinction? Does it have to do with gender? Or is "cetera" a word like "stuff," which in English we would never use for humans or house pets? My understanding is that cetera is sort of like "the remainder" or "the leftovers." Does alia only refer to people in Latin, or could alia refer to shoes and socks?
To me the main distinction between the two, as used in English, would be that we should only use etc. when the reader will not have to do the work of imagining what it entails, because it's obvious (1, 2, 3, etc.; A, B, C, etc.) -- while we use et alia in cases where the reader will not have to do the work of figuring out what it entails because they wouldn't even have any grounds on which to figure that out (Smith et al., New England Journal of Medicine). Is this related in any way to the Latin meanings?