The problem with rads + FEV = ghoul is that, why some human are spared?
Not just any amount of rads would do, ghoulification needs an almost lethal total amounts of frequent radiation doses over time (such as provided by the radioactive fallout clouds from the great war in some places), or maybe from ingested radioactive particle(s) of matter. Small or infrequent rad doses are not enough to bring down the immune system, which is the key here; Normal human immune system can and does stop airborne FEV by adapting and creating antibodies. For a virus, the FEV is a big complex multi-molecular thing with a spherical protective protein sheath (makes it radiation resistant) - This is propably why it is fairly reliably detected and resisted by a normal immune system.
(Also, radioactive fallout post Great War propably was not an even all encompassing blanket without gaps - Terrain and weather patterns play a part here; Some areas get hit hard with radioactive fallout, while some places may be left entirely untouched. Some very lucky surface survivors may even have gotten through the whole disaster with no rads at all. Same thing holds true for plants and animals; an untouched valley sheltered by mountains could allow for many species to survive the nuclear holocaust intact.)
All this is also the reason why supermutants are created by dipping into entire vats of FEV, and not by breathing in the airborne FEV that has been spread all over the planet - The sudden massive infection from the FEV vat immersion punches through the immune system with sudden brute force before it can adapt or produce enough antibodies to halt the viral invasion. Even so, the very best supermutants are made from 'prime normals' which means humans with zero or near zero prior exposure to airborne FEV, such as vault dwellers. Lack of FEV antibodies makes for more survivable, better supermutant conversion which leaves more of the subjects intelligence intact.
In this context, rads+FEV ghoulification would be more like a seriously flawed but still survivable FEV conversion, with only some of the desired genetic traits such as halted aging retained. The significant radiation doses absorbed by the ghoulifying person make the initial human DNA to be modified by the FEV too seriously flawed to begin with, hence the ghouls do not turn into supermutants ever, but may sometimes have other various mutations or health problems like for example a tree growing out of their head.