How likely is it to contract HIV through a very small scratch, if you get scratched by a used instrument?

Basically zero. I doubt anyone would say this justifies HIV prophylaxis. There's data on punctures and jabs, but nobody's connected HIV transmission to a scratch. Is the instrument itself coated fresh blood? The virus is wimpy and won't survive otherwise. If you are obsessed with the idea that you may have contracted hiv, you deserve to get a nucleic acid assay to be sure.