Sure we do, on occasion anyway. Cacti, fungi, alumni, syllabi, loci, foci, radii, moduli, stimuli, uteri, papyri, nuclei, termini.
Language isn't about being "correct", as there's no truly objective standard. Rather, it's about being understood. But I guess you didn't watch that video.
Every single plural you list is derived straight from latin. If you created a neologism for a previously non-existent concept you would pluralize it with “s” in English eg email becomes emails not emali. The “i” as a plural is only for words taken straight from Latin.
I watched it. I don’t place any value on youtube videos made by people speaking outside their expertise.
We don’t use it in English. We just retain the latin plural for Latin words. We don’t use Latin plurals for Greek words that use a different standard for pluralization.
You made an appeal to authority when you provided the Steven Fry video. Fry is not by any standard an expert on the English language or linguistics. He is an actor and he has written non-academic pieces that are not on linguistics.
There’s no reason to roll your eyes when someone rejects your non-expert source as it is an appeal to authority.
We don’t use it in English. We just retain the latin plural for Latin words.
The English plural of alumnus is alumni. Yes of course it's retained from Latin. But it's also become a word in the English dictionary. We actively use that form in English written and spoken communication. It's been absorbed into the language. I've never once heard "alumnuses".
No, we generally don't use the "i" ending on new words, but we could. There's no rule against it.
You made an appeal to authority when you provided the Steven Fry video
I never claimed he was an English professor, though he's certainly an expert when it comes to usage. I shared it because I like his point, and I think you're a bozo for ignoring it, since it makes a strong case against proscriptivism.
That gets down to what you mean by "acceptable". Octopi is not a natural derivation of octopus as the plural is octopodes just like "aint" is not a natural derivation of any form of the verb "is" BUT in both aint and octopi's case most understand those words so they might be acceptable depending.