Well, not exactly. Worf didn't become head of security until the second season.
True, but that's the route they gave him.
They didn't really know what to do with the character in the first season, indeed he was a late addition to the cast added after the first cast promotional pictures were taken, indeed he's even absent from the very first one done because of that. Really, the thinking at the time was just "hey let's have a Klingon serving in Starfleet, show how much has changed since TOS."
He was a very green ensign, learning the ropes.
Same with Geordi.
Nothing wrong with that. We saw in the Lower Decks series how officers progress.
TNG just showed it across it's entire running.
Same with Lower Decks.
Everybody naturally progresses through some field that they chose, and then goes down a specialization.
Michael didn't get the role because of his physique or that he was "built like a tall linebacker." That was incidental. Rather he got the role because when he showed up for his audition he was in character the entire time to the point where he wasn't friendly or chatty with anyone, he walked in, recited his line and immediately left without any additional chit chat or small talk, which prompted Roddenberry to jump up and scream "Him! He's our Klingon!"
Regardless of how Michael Dorn got his roll, it happened, he's a very important part of Star Trek History.
In the real world, Barclay would’ve washed out while at the Academy.
This is the "Trek World", apparently SFA is good with dealing with special cases like Barclay and helping him along the way.
Despite his personal issues, they work with him to make him shine at what he's good at and work on his other foibles as well.
That's why he was able to serve on the Enterprise-D.
It was for his talents. The "Best of the Best" all choose to serve on the Enterprise-D.
Even if there are some personal quirks like with Barclay.
You usually don't make it into Flag Officer ranks w/o going through Captaincy at some point.
Look at Janeway, she's a scientst by trade.
Became Captain of Voyager, is now a Flag Officer and decided to stay in the Command Division.
Some Officers decides that they like the Command Division and stay there.
There must be other Flag Officers within the Other Divisions:
Vice Admiral Toddman is a StarFleet Flag Officer who is part of StarFleet Security.
For a Career Officer, he must've been Captain at some point in his career before moving up to Flag Officer status and back into Security.
To date, he, Admiral Beverly Crusher, Commodore Geordi LaForge, and Commodore Oh are the only 24th century era flag officers to not wear command red as their division color. However, there is a 23rd century precedent to this practice, as all TOS flag officers wore command division gold except for two Commodores, Stone and Stocker.