Just explain it like this:
"A sword has a property; let's call it "Swordness", for lack of a better term.
An object with Swordness, no matter how it is shaped, is a sword. A fist imbued with this property will cut, and will stab, like a sword.
No matter how it is shaped, an object lacking Swordness does not function like a sword. This is an insurmountable barrier, unless you imbue an object with Swordness, which is done by imbuing the object with [Energy X], inflected to the 3rd degree."
In other words, just go through and have a section at the beginning explaining that function, and not form, is the name of the game we're playing.
I, for one, welcome the addition of sword-fists. Plus, you can have all the fun of saying:
"This functions like a sword and a particle accelerator. Yes, I know it looks like a fish, but screw you and your form-based logic."