From what I recall, the Service Corps was were some of the Inquisitors came from. That and apparently a few Temple Guards. It is also likely the younger Inquisitors could have been children Palpatine kidnapped.
His Imperial identity and position wasn't what got him caught, it was sticking his neck out to assist Paul. (Also: Sardaukar. ) He could have disappeared into the desert, never to be found again if he so chose, but instead he chose to make a difference. Well it's possible some got away into the uncharted wilds, never to be found again and as Yoda and Kenobi proved, it was possible to either avoid making a disturbance in the force, or hide within a nexus that would blind others to their presence. Plus Ahsoka was out there for years and Vader only sense her when he was all but within line of sight. Likewise Kanan hid within the galaxy's underbelly as little more than a vagrant worker. All that said, such Jedi who really went to ground with native populations off the beaten track, are unlikely to have ever resurfaced again. Seems reasonable enough. They managed to rescue those Jedi potentials from Mustafar but it's a fair bet he didn't give up on the enterprise. Plus we know the Grand Inquisitor was once one of the Temple Guardians and given the seeming ages of the other Inquisitors, they would have been about Padawan or Initiate age when Order 66 happened. The only issue I have with the notion is that the service corps seems to have been made up with what Sith would characterise as "weaklings". It's hard to see them turning such ones into hardened assassins. Artificial behaviour modification perhaps? Like a more invasive version of what was used to educate the clones. Sidious did seem to have something planned with those children that involved mental control of some sort.
Service Corps also has force users that failed to picked up by a Master. While most would have been fine to serve, I get the impression a few would have harbored anger for being passed over for someone else they didn't feel was as worthy as themselves (there are only so many Jedi Knights and Masters out there). Jealously, envy, anger....the dark side can they lead. Can fuel the force with greater power than the light side, but at a cost. Palpatine could probably sense those that could be turned to the dark side and would offer them power and training if they would be loyal to him. He would not train them as a Sith. No. But he would have them trained enough to be Jedi Hunters and be skilled at what they need to do...but no more. They would make terrible Jedi, but their jealousy and envy can be used by Palpatine for his own purposes. And of course, those that would not serve, died.
^Yeah that's pretty much how I see it. The attrition rate would have been pretty high though since relatively few would willingly turn and of them few would survive the training. I wouldn't be surprised if there were less than a dozen or so Inquisitors at a time.
I wonder if all the Inquisitors are force sensitives, or if there are those that are just really good at investigation and torture to break anyone...even a Jedi.
I think it's a safe bet that they're all force users, since hunting down other force users seems to be their primary purpose and sending non-force users to do the job seems like a bad idea. I suppose a squad of Mandalorian super-commandos could potentially take down even a Master, but they still have to find them first...and also be *really good* and well coordinated super-commandos. Conversely, if the Empire wants to hunt down non-force users, well that's what they have the ISB for. Indeed, note that Kallus only called in the Grand Inquisitor *after* seeing Kanan in action.