I've seen on this board that these are the rules: for certain ending letters - which include s - to add es to pluralize, and add only s to the rest. The rules say never change y to ies. And of course never use apostrophe!
But I have a friend whose surname is Kos, pronounced "koss". Should her family be "the Koses"? That looks like it would rhyme with the pronunciation of "the roses". My gut says double the s, but maybe not if we don't change y to ies...
http://grammarist.com/style/last-names/
Plural last names
Making a last name plural should never involve an apostrophe. The members of the Johnson and Smith families, for instance, are the Johnsons and the Smiths, not the Johnson’s and the Smith’s.
Last names ending in s are no different. Most nouns ending in s are pluralized by adding es. This applies to last names as well. The members of the Edwards and Doss families are the Edwardses and the Dosses, not the Edwards’s and the Doss’s. Your spell check might disapprove of the correct forms, but spell check is wrong on this matter. If you understandably find words like Edwardses a little too awkward, consider rewording to avoid the plural. For instance, the Edwardses can become the Edwards family or the Edwards household.
I would recommend writing the plural of the name "Kos" as "Koses". The answers to the related questions Plural of 'yes' and What is the preferred plural form of “bus”? indicate that there is no general requirement in English to double "s" when it comes after a short vowel and before the plural ending "-es" (in fact, the spellings yeses and buses seem to be preferred currently).
The answers to How do I pluralize a name ending in “y” indicate that there is a general reluctance to alter the spelling of a base proper noun when the plural affix is added. From Alex B's answer:
Huddleston and Pullum 2002 give the following rule for proper nouns: “the base always remains unchanged in both speech and writing” (p. 1595).
That said, it is possible to find examples of people doubling s to ss in the plural forms of other, more common names. Merriam-Webster says the plural of "Gloomy Gus" can be either "Gloomy Guses" or "Gloomy Gusses". So I think you may have some choice in this matter.
English spelling is often ambiguous, so while I understand the desire to indicate the pronunciation more clearly, I'm not sure that's a very convincing reason to use the spelling "Kosses". It's true that "Koses" is ambiguous, but even "Kosses" would have some ambiguity of its own: there's no way to know for sure that it rhymes with "mosses" instead of rhyming with "posses".
