I own PP Israelis and AK-47 "Professionals" which, IIRC, are roughly the same thing as their Vietnam Australians. I also own the FoW Australians and Israelis.
In general the FOW figures are much larger than the PP figures. Both are nice but the FoW figures have the edge in animation and detail. I also prefer "larger" 15mm figures. I don't believe they would mix well given their significant difference in size.
That being said, the difference in available troop types isn't that great depending on the level of combat you're playing. We use anything from a mech platoon to an understrength company per side and both ranges can support that with appropriate platoon and company level GLs, LAWs, MGs, and mortars. Go above those levels and both ranges will have gaps.
In any case, as fastolfrus notes, any of these figures will work for "generic" moderns, especially the PP Vietnam Australians and FoW Australians. We primarily play near-future games and use the figures above with zero issues. In 15mm with 1 stand representing a squad they work perfectly well.