But is it safe to assume they can regenerate skin?

I don't think they really regenerate... Just seem to keep on living after they should be dead.

Argh... well, is it too much of a stretch of the imagination to think somewhere along the way, a particular human mutated into a ghoul, and gained the ability to regenerate limbs?

If you want to make a mod story that has a creature that does this, go for it. He could certainly be a special ghoul, maybe a starfish fell in his FEV vat and he absorbed the power to regrow missing limbs.

Ghouls are irradiated people, not super mutants. FEV makes super mutants, floaters and centaurs, not ghouls.

Ghouls are irradiated people, not super mutants. FEV makes super mutants, floaters and centaurs, not ghouls.

Well it's not clear whether it was radiation or FEV that mutated Harold. It's not far fetched to assume that the FEV vat chamber that Harold passed out in was also irradiated; in fact that's more likely since I don't see how the FEV could have affected him since he didn't fall into it like Richard Grey did. The FEV in Mariposa was a liquid; I doubt that the FEV evaporated thus becoming airborne since it was still there decades after the war.

To the untrained eye, Harold appears to be an ordinary ghoul. This is not so, however. Ghouls are the result of massive radiation damage to a human body: Harold is a product of the Forced Evolutionary Virus. Unlike most people who are exposed to FEV, he did not become a super mutant, but is the result of a unique combination of radiation damage from constant low-level exposure, indirect exposure to FEV (It is unclear how Harold was infected with FEV as he blacked out for a while) and a fair amount of random chance. He's not a ghoul, and he's certainly not a super mutant. To quote Tim Cain, "Harold is special."
Furthermore, Tim Cain has this to say on the subject: "As for contact [with FEV], any contact at all will infect the subject, but the amount of contact determines the result. For example, I imagine Harold had some contact with the virus, but he was not fully immersed in it, so he became a different mutant than the Master's subjects. Full immersion, of course, is the preferred method of infection, as it provides the virus a large surface area for infection."

It was the FEV... He only became the harold we know after a visit to Mariposa.

Like I said though he didn't fall into any vats, he just collapsed. He was more likely exposed to radiation than FEV.