When I was taking high-school English, the texbook usually set out these guidelines:
–Ordinarily a noun ending in -o takes -es as the plural.
–If a vowel precedes the final o the -s plural is used. (radios, rodeos)
–If the word refers to music or other fine arts the -s plural is used. (solos, virtuosos)
But I was composing text in Microsoft Word and I used the word “aficionadoes,” following this rule. The spell-check program stopped at this word and claimed that -s is the proper plural. Does Bill Gates have his own rules for plurals?
When a word is taken directly from another language, usually so is its plural. Like solo and virtuoso, aficionado is taken directly from another language (Spanish), and so is aficionados, its plural.
Oh.
Beggin’ your pardon but solo and virtuoso are taken from Italian (even though Spanish may have the same words, I don’t know) as descended from Latin. The proper plural in Italian for these is soli (which you will see in musical notation) and virtuosi. Nearly all musical terms in the US are taken from Italian (aria, soprano, crescendo, staccato, etc.).
When it comes to English spelling there are no rules, only guidelines. Your third rule probably comes from the fact that just about every word in music and fine arts in English that ends in “o” comes from Italian. In such cases we add an “s” by convention. Probably the same with words taken from Spanish (which includes rodeo, BTW), although perhaps Spanish also ends its plurals in “s” (I don’t know, I don’t speak Spanish).
I once had a discussion with someone who held that this is true only if the original word doesn’t take us east of Vienna. Thus, kimonos, safaris, and kangaroos. (I’m guessing on these, I don’t know how to form the indigenous plurals)
Spanish’s plural is usually made by adding “s”. (or “es” if the word ends in a consonant)