i'll say law's sig attack was revealed against smoker while kidd had apoo
A Signature Attack is the one where you Charge your own Power in it & it Scales with your Ability (As i said, basically their Hiken)
Law's DF is about creating Rooms, what happens inside those Rooms is up to him, actually those Effects aren't Fixed, it's just that He fights like that cuz He is a Doctor, but his Main Ability is making Rooms
So his Signature Attack is when He charges something with Room & unleash it as a Finisher, which is what Shock Willie is, it's basically his El Thor Attack, the Rest are like his Vari Attacks, as for Kid, his Power is Magnetism, not just Metal, meaning his Signature Attack is when He charges his Magnetism & unleash it similar to Hiken or El Thor or Sables or Dai Funka or Raging Tiger or Ursus Shock ... etc & that's what Damned Punk was
Signature Attack represents a Character's True Power, He can use Gatling Version of it or an Ultimate Version of it or take a Form where you're Passively at Max Power, the rest are just Variations of those Techniques (Just like Rooftop Luffy for example, He has Roc Pistol, Roc Gatling, Roc Bazooka & have Gear 4, these are all you need to know how Strong is that Version of Luffy, the Rest are just Variations & there are Endless Possibilities for what Oda can draw)