I don't have any of the old Revell Prussians, but I do have plenty of Austrians and they seem quite compatible with HaT's Prussians, as are Zvezda's grenadiers. Certainly within the realm of reasonable human variation, so I don't think this should be a major concern.

I would however be very happy if any separate heads/hats in HaT's Austrian sets were interchangeable with Hat's Prussians, which would be ideal for conversions since in the Reichsarmee there were quite a a few units wearing Prussian-style uniforms but with the grenadiers wearing Austrian-style bearskins.

Exactly - great idea, and maximises the use of the HaeT figures

sure more firing figs.