I think it's debatable, not that The Knight of Ren are the former Jedi trainees/Padawans, but rather in the sense that I do think the Knights of Ren served Snoke some purpose, but I'm not sure if they are really his personal body guards since The Last Jedi introduced Praetorian Guards to play that role. If the Knight's of Ren were his body guards, then why were they not in the throne room???
In Addition The Knights of Ren seem more like a religious cult or military order not unlike Star Wars' own version of Knights of erRENt (Chivalrous Errand Boys) or "Ren" could also be short for REN-aissance suggesting a something like a revival or more literally "rebirth", which would go with the undisclosed mythology (Kylo's and Rey's raw power and force dynamics or concept: The Force AWAKENS)) and Kylo Ren's lightsaber design
The lightsaber, based on an ancient design dating back to the Great
Scourge of Malachor, contained a cracked Kyber crystal, requiring it
to have lateral vents on either side of the handle in order to divert
the extra heat generated by the crystal.
Many people speculate that The Knights of Ren may be an advancement/evolution/expansion on Acolytes of the Beyond, since Kylo Ren's (and possibly Snoke's) obsession with Darth Vader and/or the Skywalker bloodline matches with the ideology. (ie: "That lightsaber, it belongs to me." - Kylo Ren to Rey TFA )