Just to add my 2 cents in on this...
I always wear 2 pair of socks, one liner one wool or smart wool, even fleece on occasion. I've never had issues wearing two socks, as long as I wear one liner, one thicker sock over. Now two thick socks, I absolutely hate. The trick for me seems to be that the liner sock is thinner than the outer sock.
The liner sock should be synthetic, NOT cotton. Both of these layers pull the moisture away from your feet. The outer wool will wick moisture out but in the neoprene bootie it's got no where to go. Even though I wear breathable waders, the neoprene feet are not breathable, hence damp socks when I remove my boots. Now they're not SOAKED, but they are damp on the outside, inside I find I'm usually dry---down to the liner.
Acristick---check out the CoolMax liner sox from Sportsman's Warehouse or Cabelas. They are very comfortable and I've found that they keep my feet dry and cool. Then put a pair of wool or smart wool on top.
I've never had issues wearing two socks. The thin liner sock is synthetic, NEVER cotton as that will absorb moisture and thus keep your feet colder.
Regarding the more vs. less air space in the boots...I prefer more room than less as it was recommended to me by the local fly shop I frequent. My boots are up one size so that I can wear two socks and also so that I'm not cramming my feet/socks into boots. Similar to the concept I'd suppose to why tight clothing in the winter is colder.
The warmers that go inside boots do sound interesting, that might be something to try in the near future.