the only difference between G17 and G18 concerning holsters is the switch. don't know if it will fit a 6004 though but i guess it will.
BTW. the ksc g18 does fit a blackhawk CQC serpa holster perfectly.
EDIT: btw very strange:
I had a KSC G17 & G18 both with metal slides and both fitted the Serpa holster.
A friend of mine had a KSC G17 with an ABS slide and it needed alot of pushing and tugging to fit!