An interesting case of this: Genma addresses Akane as
'Akane-kun'. However, he addresses Kasumi as
'Kasumi-chan'. Nabiki...I don't think he ever
addresses Nabiki directly, actually. I can't say
I blame him. ^_^

He does, actually, in the OAV where Ranma and Akane have their cook-off.
He calls her "Nabiki-kun".

The Eternal Lost Lurker

-Richard