[quote=@Sombrero]
Are weapon martial arts included in the 6 martial arts you learn? Do treatises and extensions to weapons count if a certain otherwise unarmed martial art has them? Or do people who know weapon martial arts in general sort of mesh over to lesser degrees out of experience fighting other weapon users?
[/quote]

yep you can include specific weapon martial arts but if you don't you will still be proficent in the 'Tengakure' style of weapon fighting.

I'd go with the general meshing to lesser degrees of fighting.