Nice and interesting! I like the use of the MWDA Centurion figure.
As for flyers, have you considered the drone variant of GZG's Mantis VTOL?
http://shop.groundzerogames.net/index.php?_a=viewProd&productId=1489I quite like it, and although it is 15mm, I think it would look okay next to your 20s, especially as a robot/autonomous drone thingy. Obviously, it's more in the weight class of the Mk1 Hunter Killers in the third movie rather than the big flyers from the others, but it could be a proper start.
Also, my local Modelzone recently sold off Terminator RC helicopters, including some small (6") hunter-killers. They were about 15 quid then, and may have gone down since if a store near you does the same. Didn't look closely when I was there on Saturday, so they might already be gone, but they looked nice enough, although it might be considered a shame to forego the actual flight capacity to use it as a wargame piece.