The campish version of Masked Rider and Big Bad Beetleborgs are not canon in PR. MR is a television series and Beetleborgs is a comic (the fact it's also a comic in their reality is completly irrelevant). The Turbo/Metallix crossover in the comics, could, by extension, just be akin to the PR world's answer to or Superman/Ali or any other "superhero meets celebrity" comic.
A Dex DOES of course exist in the MMPR timeline, but he has a much darker story and Edenoi is a planet on the verge of ecological collapse. He's sent to Earth because it's going to die any minute
When Dex fled Edenoi and settled on Earth, the events of the Masked Rider television series still happened, but he then turned his whole experiance into an autobiographical kids show starring himself :buttrock:
The only reason anyone wants to discount RPM is because if it IS canon (and it IS), then PR ends on a "bad wrap"..we know by the year 3000 the human race have abused Zordon's legacy and become entirely toletarian, racist, and willing to perform genetic experimentations, leading to Ransik's rise to noterierty.
It's a sour reflection of the human race. An unavoidable tragedy, it says everything nasty about us, and then leaves you to think about making a difference to prevent that in your own lives.
...But there's a far worse fate than that. Chronologically, this would mean "Reenforcements from The Future" is the final episode of Power Rangers...and we all know that ended with Shayla's "Ode to Nature"
...Ouch.