Notably, Fallout (as a franchise) has a lot of post facto development. The whole notion of the Vaults-as-experiments was largely introduced in Fallout 2, after all. If Fallout says that ghouls were products of huge overexposure to radiation, Fallout 2 hints that maybe some FEV is involved, and Fallout 3 says that there is also a mysterious X-factor (which is how they get the Lesser Rad Orb power, I guess), it's quite possibly just another case of post facto design: "No, wait, you thought it was this, but it's actually this other thing!"
I have a strong suspicion that FEV was not central to the ghouls in FO1, and here's why: FEV is an important plot point; the dip that the Master uses to create Super Mutants is a type of FEV cocktail. If FEV is that central to the story, then having another group that's FEV-affected -- the ghouls -- would mean that this connection should be somehow important. Dramatically, you can't just drop in a group that has some sort of tie to the central "prime mover" of the story's villains and then fail to explore it. I don't recall ever seeing any cut documentation for design of ghouls that would talk about FEV or their connection to the whole evolutionary project -- even as a failure or unintended side-effect -- which means that when the ghouls were first conceptualized, they probably didn't have an FEV connection. Instead, the importance of the ghouls is partly symbolic (as a prior poster pointed out, they show the burned-out remnants of humanity living among the burned-out remnants of the world) and partly as a moral dilemma for players who are given the choice to steal the source of the ghouls' fresh water and thereby placed in the position of pitting the interests of the home (Vault 13) community versus the community of "subhumans and monsters." The ghouls can stand on their own in that regard -- and the fact that they have a society shows that they are a self-contained story element; in a sense, ghouls are like the ultimate victims in Fallout: They are always getting a raw deal, first from flesh-rotting ghoulification, but then from Super Mutants, "smoothskins," even the damn trees. If FEV were responsible for ghoulification, then, it would be important to explore this plot thread. Since it was never explored, even in any design doc that I remember reading . . . probably wasn't an initial impetus.
That doesn't make it any less valid now, of course, if you can find an interesting way to tell a story about it while retaining internal consistency (in the same way that Zelazny does in his Amber novels, where it seems that every few chapters he turns your expectations of the world upside down while still retaining his own consistency).