Well for one thing Projekt X figures aren't 28mm, they are 36mm, or 1/48th scale. So they aren't even designed to match those other ranges. The fact that they are on big display-style slottas doesn't help matters either.
Having said that, with the more monstrous figures it doesn't really matter. Who says exactly how tall a werewolf is supposed to be? But the ordinary humans might be more of a problem.
I have a bunch from this range and will be using them on smaller bases with my 28mm figs, I think they fit in just fine. But then again I don't have any of the humans, just the abominable creations of Nazi SuperScience.