Rapid Fixer concentrate is basically 60% Ammonium Thiosulfate (that's as much as will dissolve in water reliably) with some buffering agents, i.e. the sulfite and hydroxide that that recipe calls for. You may or may not need to add the EDTA, but B&W Rapid Fixer should work with C41. The lower concentration recommended by that recipe is because C41 has a lot less silver to strip out than traditional B&W film; the density is produced by dyes instead.
Don't use sodium thiosulfate, it will be a far slower fixer.
Edit: sorry, it depends which one you buy. Most rapid fixers are acidic and the buffers included with them differ. Ilford sells both Hypam and Rapid Fixer with the same thiosulfate content in each but different buffering arrangements. Only the Hypam is compatible (due to a different pH) with hardeners, which means that one or the other may or may not be compatible with your use of EDTA.