The rocket launcher sound replacement method won't allow you to use custom sounds in pure, although you can probably lower the volume of the default sound by changing "volume" or "soundlevel" in the scattergun weapon script.
If you're using weapon scripts for custom crosshairs already, you can edit it right in there, otherwise the script can be extracted from tf2_misc_dir.vpk (it's in the scripts folder and called tf_weapon_scattergun.ctx) then decrypted with CtxConverter/vice. At that point, you can edit it like a text file (change volume or soundlevel) and then put it in tf/custom/whatever/scripts/tf_weapon_scattergun.txt and it'll override the built in one.