I believe that Rodenberry had said that Starfleet is all officers, but we see engineering personnel wearing red jumpsuits with Scotty being the only one in uniform. Then in the TOS movies, we see people in the background wearing the jumpers we first see in ST II but with black dickies instead of the orange, All of those instances I took those people to possibly be enlisted rates. And we know that O'Brien is a Chief Petty Officer, an enlistedman.
There's a lot to unpack in there. Roddenberry contradicted himself with that assertion, as he had a CPO Garison sitting at Communications in "The Cage". It's there in the script, that he wrote, as well as in the end credits.
Regarding the movie uniforms... Bob Fletcher, for TMP, had something similar to later TNG, where there were multiple uniforms available per person, depending on what they were doing. Class A and B two piece uniforms for regular duty, Class C one-pieces, and "scrubs" -- utility one-pieces with a tied belt/sash. There was also uniform color -- white for command personnel, blue-gray for senior staff, beige/tan for other personnel, rust/brown for techs, etc. And then the department color on the insignia patch on top of
that. But there were definitely enlisted personnel
there, too.
Starting with TWOK and going through at least "Yesterday's Enterprise", the TMP jumpsuits were altered a bit and re-dyed for enlisted personnel and trainees, while officers and officer cadets got a revised version of the TOS uniform, plus a new jacket.
The protective suits are worn by most engineering personnel in the first six films -- officers and enlisted. Gotta look at the rank insignia, there. The red collar/dickie versus black does distinguish active-duty from trainees and cadets, though, yes.
About the only place Starfleet is shown as "only officers" is the three seasons of TOS, bracketed by the first pilot and the first film, both penned by Roddenberry, where there are very definitely enlisted personnel.
But O'Brien's a goddamn Lieutenant. He was an Ensign in "Encounter at Farpoint", and then was in the Lieutenant grades, and addressed as such, and Captain Maxwell spoke of him being his tactical officer on his previous command, all the way up until "Family" when Ron Moore couldn't tell the difference between
job (Transporter Chief) and
rank. I just wanna grab him and shake him and ask him if that means the
Chief Engineer or
Chief Medical Officer or Security
Chief are all Chief Petty Officers,
too. Idiot. I love the idea of there being a crusty old sergeant on DS9, but it shouldn't have been such a sloppy and insulting retcon.
As for shields, what I find funny about them is how when they are fresh, the percentage of damage is pretty high and they start dropping pretty quickly, But when they start getting low, somewhere below 50% they seem to become more efficient and drop more slowly. So, instead of dropping in increments of, say, 10% each hit when above 50%, when they get below 50% their shields start losing power in 5% percent, or less, increments all of a sudden. It's like shields at 90, 80, 70, 60, 50 percent, then it becomes 45%, 43%, 41%, and so on down.
They're meant to ablate with the incoming whatever, and then have time to bounce back. When ongoing damage is coming in, they reroute power from less-vital systems to shields, or, if the damage is focused on a particular area, they'll reroute power to the shield sector in question. Either way, dumping more energy in to try to stabilize or "harden" the shields, regardless of how low their integrity is getting. Like swapping out a one-inch iron plate that's been deformed by an incoming round with an eighth-inch stainless steel plate because that's all you have left. The iron dissipated the ballistic energy by deforming, at the cost of its integrity. The stainless steel is harder, and if the shot hits at the right angle it'll ricochet off, but more of the kinetic energy is going to get through to rattle you around. You can take a forty percent drop from the first hit, as it gets distributed through the system and you prepare your response. But beyond that...
It's as true in Star Trek as in real life. The best defense is to not be where the blow falls. Between evasive action for an active enemy, and active and passive deflectors for natural micro or macro objects, the intention is to prevent whatever it is from actually reaching the ship itself. When energy shields were discovered/invented, it was like getting extra layers of hull or armor without the mass of all that extra metal. But it still means the primary defense has failed. If you can keep it, then, at least a glancing blow, and give the shields time to regenerate after impact, everything should still be okay. Below that, it's as I describe above -- attempts to minimize/mitigate damage. Maneuver the ship to present fresh shields to the enemy fire, say, or cut power to life support to try to keep shields up a little longer.