Jesse, Fives, and Echo’s ARC training - Cody and Rex, too - probably included teaching them how to get information from people who didn’t want to give it. Roughing up a suspect. Encouraging someone to talk. Enhanced interrogation. Torture.

ARC troopers are supposed to go solo for long periods of time, often behind enemy lines. They’re trained to be creative and independent, perfect for jobs with an open-ended brief. Clone commandos are for fast infil/exfil with a clearly defined mission and measures of success. If you need to train the locals in CIS counter-insurgency, send in an ARC. If you need to have someone assassinated, send in a squad of commandos.

After Echo’s death, an angry, grieving Fives could’ve been sent into a high pressure volatile situation for weeks or months. Same thing could have happened to Jesse after Tup’s and Fives’ deaths and Kix’s disappearance. 

There were torturers in the Cuy’val Dar who could have passed on their techniques; the Nulls, the Alpha ARCs, and the commandos were all taught how to resist pain. The clone commandos at least weren’t taught interrogation techniques, but they have internalised its use enough to be okay with leaving the torture to other people:

“We’re not trained to interrogate prisoners,” said Niner.

He put it out of his mind with a conscious effort, and not only because Ordo had insisted on undamaged prisoners. He knew how to kill, and he knew how to resist pain, but he wasn’t sure how to inflict it deliberately.

But he was pretty sure that Vau did. He’d leave the job to him.

Vau is Walon Vau, who tortures someone to death for information in RepComm. He’s one of the Cuy’val Dar, responsible for training some of the commandos. He explicitly tortured them as part of his training program to the extent that one of them wants to kill him for the things Vau inflicted on him, mentally and physically.

Niner was trained by Kal Skirata, no Vau. Skirata trained the Nulls in interrogation, so he could have used those skills to teach his commandos how to resist pain. Or there could be another torturer in the Cuy’val Dar, which seems likely given the hit rate so far.

Nulls have been trained to think of interrogation as “having a chat” with people, when those people are heavily wounded, are being denied medical treatment, and have a gun to their heads:

...Ordo said. “You can cooperate or not, but I might as well execute you now if you’re not going to be useful. We can’t handle any more prisoners.” It was always hard to tell if Ordo was playing the interrogation game...

The original Alpha ARCs were considered to be uncontrollable, and were ‘pretty much raw Jango’ after the Kaminoans overtweaked the Fett genome and produce the Nulls. They were trained personally by Jango Fett.

 Alpha-17 tells Ventress he’s been trained to resist torture when she captures him and Obi-Wan Kenobi; Jango Fett could have done it himself, or gotten Vau or Skirata to do it. In that environment, it’s hard to believe that they didn’t teach ARCs any ‘enhanced interrogation’ techniques. It was normalised enough that the commandos don’t question handing over their prisoners to be tortured, even if they didn’t know how to do it themselves. It’s entirely probable that Jango Fett thought it would be a useful skill for his Alpha ARCs to know - the way they’re deployed means they might run into anything out there.

After Geonosis, some of the remaining ARCs and some of the Nulls start to train more clones as ARCs; this is probably when Cody and Rex went through. They’re line officers, though; they would never operate independently in the same way an ARC is supposed to be able to, although they could. The training regime might also have gotten more brutal as the Republic tilted harder towards the Empire in the later stages of the war, and people like Tarkin came to power. 

Clone troopers of any kind don’t get mental health treatment. They get reconditioned. But if you did that to an ARC, they’d lose the initiative and creativity that makes them so effective, and waste all their training. Maybe an ARC that’s too close to the edge gets redeployed back to their original unit for a while, so they’re surrounded and supported by the troopers they know and love. Like Fives, after the Citadel, on Umbara. Or Jesse, after Kix and Fives and Tup, on Anaxes. So they don’t snap completely, and they’re still being useful to the war effort.