There is no FAA requirement for SMS in pt135 ops, or for pt91 either. SMS is only effective if management believes in it. Otherwise it is just an additional worthless paperwork exercise for flight crews. I flew for a company that did lots of international flights, which SMS is a foreign requirement. The owner used it as an advertisement for business but rarely would follow the program. Frat scores are a useful tool but again they can be manipulated if management doesn’t believe in them. I don’t think even the FAA likes the SMS idea or they would have made it mandatory for everyone long ago. The only reason pt121 is mandated is due to international ICAO requirements just like SIC type ratings are.