It’s hard to tell because the deviating form in Latin is actually the nominative singular, which is why vocab lists include the genitive singular as well. All other forms have the same stem aside from Nom. Sg. A few examples are:
senex - senēs (elder)
rēx - rēgēs (king)
index - indīcēs (index)
So really anything could work as long as it ends on -ēs in plural and starts with kleen-.
Kleenex is Kneenes according to the rules of Latin, actually
But the plural of index is indices in Latin, so shouldn’t the plural of Kleenex under those rules be Kleenices?
It’s hard to tell because the deviating form in Latin is actually the nominative singular, which is why vocab lists include the genitive singular as well. All other forms have the same stem aside from Nom. Sg. A few examples are:
senex - senēs (elder)
rēx - rēgēs (king)
index - indīcēs (index)
So really anything could work as long as it ends on -ēs in plural and starts with kleen-.