^I don't agree. Whether the were in costume or not, all five core Ranger actors were members of RPM's regular cast from the very first episode. In the other three examples, only four of the current or future Rangers were present as regular cast members from the beginning of the series, with additional Rangers only joining the cast later. In RPM, there were five team members
from the very beginning; it's just that the more serialized nature of the show meant that the arc of the team's formation took a bit longer than usual.
And as I said, in RPM Doctor K has five Ranger suits ready to go from the beginning and just needs to fill them all. In the other cases, the subsequent Rangers have at least somewhat separate origins from the initial three. Even if they ultimately get their powers from the same source, in no other case are we shown that five slots are ready to be filled from the beginning. So while RPM doesn't exactly fit the usual 5-Ranger formula, it's closer to that than to the usual 3-Ranger formula. (And heck, RPM doesn't exactly fit any standard Power Rangers