"Captain Kirk is a fascist."
To reach this conclusion, one has to conflate all exercises in force as being fascist. That's nonsense, prima facie. (Although we do see this nonsense being argued in real time, WRT some of the cities which have seen protests devolve into riots.)
The vast majority of Kirk's uses-of-force was in self-defense OR to enforce a rule of law (or rule of morality). That the former is NOT fascism is self-evident; one could only argue that had the Federation not been in that particular place at that particular time, no force would have been needed, but that "reasoning" would have, taken to its logical conclusion, require Earth to eschew space exploration completely. (Interestingly, the sequel series Star Trek: Enterprise explores the possibility of future Earth isolationism in some detail, and devotes two "alternative timeline" episodes completely to a Fascist-Earth possibility. )
Of course, the latter (use of force to enforce rule of law or rule of morality) is more of a sticky wicket, because it requires us to decide if a law is moral or not. I wouldn't consider Kirk's uses of force to protect the underprivileged to be fascism - helping those who can't help themselves is generally considered to be a virtue.
But from there, the wicket gets stickier. Is a particular law moral or is it fascistic? If the former, then using force may be justified (depending on level and circumstance); if not, then it's not.
Now, on to Heinlein.
"Starship Troopers is an example of a movie that’s better than the book, which was written by renowned sci-fi author Robert Heinlein. The book is a political essay masquerading as a young adult space adventure. It argues that violence is “the supreme authority from which all other authorities are derived.” That’s a tattoo for a fascist if ever there was one."
Uh, no. The movie is shlock. The book won the Hugo Award in 1960, and the movie removed almost all the "meat" from the book, specifically detailed discussions on various political controversies of that day.
Heinlein, who many/most consider the GOAT of the early sci-fi era, developed from liberal roots into a libertarian of the "government-as-a-necessary-evil" sort and a bit of a prepper. He actually wrote a nonfiction book called "Take Back Your Government" which I think both "sides" in today's equation would read and nod their heads from time to time, the political right a bit more than the political left.
That all said, he was also an atheist of the humorous "poke fun at" variety, meaning that he had no problem with end-justifies-means reasoning.
Heinlein's libertarianism caused him to be an anti-collectivist of the highest order; any governmental or social structure that would dictate to the individual, outside of that which was undeniably necessary, must be opposed with all due vigor. That is a self-defense motive, not a fascist motive, and it appears often in Heinlein novels, although it mutated in his later novels away from ST's militarism to prepperism, where characters (Lazarus Long being the most ubiquitous) would simply excuse themselves from society when society became too onerous.
And yes, I think Heinlein would agree that the moment that people actually step outside our solar system, they'll find that alien civilizations are not as civilized as we would hope, and a few photon torpedoes are required to prevent us from having to isolate ourselves on Earth. That idea is controversial and speculative, but not fascism.
Think I'll re-read ST. It's been a few years......