I'm not sure but I think Iron Guard primer is an old fashion alkyd primer. He likes it because its easy to use with no recoat window to contend with and is easy to sand between coats. Epoxy primer has a recoat window, contains isocyanides and doesn't sand very good. There is a night and day difference in the quality of these two primers. I'm sure any body guy would rather use an alkyd primer if it was any good. They put up with the epoxy because its so much better. Then today most automotive finishes are geared to be used over epoxy primers so you would be bucking the system to use something else. The primer is the foundation of the finish and if you start with junk, it really doesn't matter what you topcoat it with. Both systems will work but the epoxy primer will last much longer and will protect from rust better. I believe if I were going to use Iron Guard primer I would continue with an alkyd enamel topcoat rather than spending the money for a better topcoat.
I have no doubt Iron Guard primer fills better because epoxy primers are not formulated for that purpose. The epoxy primer is put on first and if filling is needed a filler primer is applied over the epoxy which is sandable. A CNH makes a gray primer or they also make a medium yellow primer that would be better for yellow however urethane covers pretty good so it should cover with two coats.