Bear in mind that all my Latin comes from Catholic school...
Lots of people give their magus names as " ex .". This is not really appropriate; it should be . A very detailed explanation of Hermetic naming by Mark Shirley: durenmar.de/articles/naming.html
Salve = singular; salvete = plural
Sodalis = singular; sodales = plural
Summa = singular, summae = plural
Tractatus: I've seen "tractatii," "tractati," "tractii," and even "tractatem" (huh?). Plural for tractatus is simply tractatus.
Corpus = singular, corpora = plural (hence the term "corporation")
Plural for "domus magna" is "domus magnae."
Plural for "magnum opus" is "magnum opera."
More to come!