I've often wondered why Rand was at the Transporter in TMP since during the days of TOS her assignment was as a Yeoman, possibly 2nd or 3rd class Petty Officer, which is in the Command section
Well, she wore red in TOS, so that's Ship's Services.
and is an Admin/Assistant role. We must assume that she started taking Officer training after TMP but still serving as an NCO, as we've seen her as this until TUC, until we see her as a Lt Commander on the Excelsior but there she was in the Science division(?).
She was Communications Officer, same as Uhura, so same colour division. (In fact, in "In Thy Image", which morphed into TMP, she was Uhura's relief at communications, taking a stint on the bridge as part of her training.)
During her enlisted days did she also specialise in Transporter operations as well or was this something the Producers decided to do with her just to give her some screen time?
Gene Roddenberry never worried about ranks too much. He often said that rank in Starfleet was "more of a job description".
There's a memo in "The Making of ST:TMP" about working out Rand's rank. Originally, they just put metal trapezoidal insignia on her uniform's epaulets, rather than diagonal rank stripes, and called her a transporter chief without giving any thought as to how one becomes a transporter chief. Presumably, after she left the ship in Season One and retrained.
Similarly, when Grace Lee Whitney made a cameo in ST III ("Woman in Cafeteria", but playing Rand at Nimoy's invitation), they gave her a uniform top that happened to have commander rank pins on it, and command white undershirt. No one thought we'd be making freeze frames to study her rank pins decades later. It was a throw-away cameo, filmed at ILM, after the principal photography was long completed.
There's an interview in Starlog where Grace Lee Whitney discusses how they gave her a uniform for ST IV (modified from a ST II cadet uniform) and
asked her what rank she wanted to be. She said, "Well, I was a Chief in TMP, so maybe I'm now a Chief Petty Officer." (Which, IIRC, is a demotion to a non-comm rank, isn't it?)
And, of course, she then had officer rank, again, in ST VI (after winning a ST convention audience poll over Bruce Hyde and John Winston!) and "Flashback".
There was no one sitting down working out Rand's career path ahead of time.