I think that any shield should be able to costume any other shield.
This may not be as good of an idea, but it could be cool if any weapon of the same type should be able to costume that weapon. Like flourish/snarble barb, the cailbur line, the troika line etc... would all be their own "type" so that you can't costume a flourish with a troika or something silly like that.
That might be a little too complex, but I mean, we do have certain armors that can only be outfitted with certain accessories. It could work something like that.
I think at least the shield costumes make sense. :T
What do you guys think?
Some weapons are kinda smilar (a calibur and a brandish) But imagine using a troika as a flourish O_O