Sam,
you say "If subfusc is derived from the Latin "subfuscus" surely the plural must be subfusci", but I think it is more a case of a corruption than derivation. Idiom and usage beat strict grammar hands down. For example, the plural of "stadium" is "stadia", but how often do you hear that used? The word "agenda" is itself a plural - things requiring to be done- but how often do you hear "agendas". Oh, and I said "for example", but then gave 2 examples because saying "For examples" would have seemed odd.