I kind of think that weapon accessories (which I love as an idea, by the way - I made a comparable suggestion awhile back that weapon UVs could have visual indicators in a similar vein) need to follow the pattern established by armour accessories; that is, the items themselves are universal, but not all weapons would support all accessories. I think this is pretty much what Starlinvf's suggesting - one set for firearms, one for swords, one for bombs, but with limits to their applications within those groups.

Also, it's worth considering that bombs, certain firearms, and lower-level swords have relatively small models; I suspect most could support one accessory, two tops, before getting overly cluttered. That's fine by me, but.

Edit: changing the shape, colour, size &c of a projectile's not a good idea - the current projectiles are pretty clearly codified to tell players what kind of damage they're going to do, and messing with that hurts readability.