imaparasite, Blake says somewhere in Marriage of Heaven and Hell that you must know more than enough to know what is enough.
I knew someone would pull out a dictionary and babble on. I told you to consult Fowler’s Modern English Usage. Yes, both plurals can be used, but one is preferred usage. Some Latin nouns have converted, so to speak, some have not. Some are not even Latin nouns, like factotum. In the case of forum, the proper usage is forums, but fora is also grammatically correct.
In the case of symposium, both usage is fine, Fowler actually says symposia is preferred but a number of other stylists, too numerous to mention, maintain symposiums.
The matter is also heavily complicated by considerations of topic, prior use of foreign expressions, neologisms, etc, etc. As an immigrant who studied English grammar and usage very seriously, I find Americans don’t know a thing about English. To digress, almost no one knows the difference between mutual and common, and so on.