This is something I was playing around with a few months ago using Empress Miniatures helmets here. Looking back at them again the helmets didn't quite work, they looked more like they were sitting on their heads rather than being worn! So I replaced them with cut down Victoria Lamb head swaps. I realised after I'd done 2 or 3 that maxi-mini resin heads would have been a lot easier! Doh!
I green stuffed the gaps and I think they will look ok, they also got some imperial eagle cap badges. They're going on some resin tree trunk bases that are on there way, so might not be complete for a while, but they were something to do in between waiting for glue to dry and priming the Praetorians I've just started painting.