Probably half the words in English were borrowed from Latin or its descendants, French, Italian, and Spanish. Today English is hardly recognizable for the Germanic language it is, cousin to German, Dutch, Swedish, Norwegian, and Danish.
Originally, the plural forms of Latin nouns were borrowed along with the singular forms, so that the plural of abacus was abaci, of cactus, cacti. As time passed, however, that has changed in bewildering ways.
Abacuses and cactuses have all but totally replaced cacti and abaci, and foci is used as a plural of focus only in academic institutions. All dictionaries now list the plurals of callus and sinus as calluses and sinuses as the only options.
On the other side of the coin, most speakers don’t even know that agenda and media are, in historical fact, the plurals of agendum and medium. (Radio is one medium.) More and more I hear “a phenomena” presumably spoken by people who don’t know that the singular is phenomenon.
Gigi Marino, Editor of the Bucknell Magazine tells me she is weary of reading “I am an alumni” in letters to her office. The plural of the Latin word for “pupil”, alumnus, has not changed and is not even in the process of changing. The plural of this word is not optional but only alumni. It is the plural, not the singular. “I am an alumnus,” is the only way to express the singular sense of this word.
I suspect the reason for the plural of this word taking over the singular is the awkwardness of the expected change, alumni > alumnuses. No matter, the only plural for this word is alumni.
One final note. Not only did English borrow the Latin word for “pupil” as its word for “alumnus”, it borrowed the feminine forms: alumna and plural alumnae, pronounced [ahlumnee] to refer to female alums. Again, alumnae is the only plural form of alumna.
So, what if we are talking about several graduates, some men, some women? The general rule in Latin and all related languages is that in the general form covering both genders is the masculine. So alumni may refer to several male graduates or a mixture of male and female graduates. Also, if you are not sure whether the alum is male or female, alumnus is the general term to use. (This a grammatical rule that has nothing to do with sexism, by the way.)