Howdy Cliff,
SASS's Classis Cowboy is a joke. Initially, it was called Classic Shootist or something of the sort, if I recall correctly. That lasted maybe a year, then (with a alot of East Coast imput, if I recall), it was changed to Classic Cowboy and the emphasis was placed on having a "cowboy" appearance or as you put it so well, "Hollywood Cowboy Gunfighter Lawman Dandy" appearance. (I wouldn't mind it if they did a decent Hollywood outfit, but most of what I've seen the last 5 years+, wouldn't even pass for that.) What was meant to be a big bore category turned into 45 colt, 165 grainer category and there were some outrageous "interpretations" on how to wear the mandatory gear which caused further rules to be added regarding the gear. I left participation in it a long time ago and prefer the black powder categories now.
As for knives; there are lots of knives out there that can fit for SASS; look for 6-8 inch blades (fairly comfortable to carry; bigger gets a bit un-wieldly). SASS isn't period correct so there's plenty of lee-way. Your Buck knife (even though not period correct) would be fine at a SASS match. (NCOWs does strive for period correct, so be aware, knives for SASS use may not comply.)