They mix in quite well. I actually have some Asgard/AA orcs and Chronicle cave goblins in the painting queue for use together in Mordheim (I'm using some Grenadier/Forlorn Hope and Dixon goblins too, but those are bigger).
This WIP shot might give you an idea. There's an Asgard/AA black orc at the front (with the green shield). Around him are Chronicle hobgoblins (much bigger than the cave goblins) and a Dixon goblin, but you can just make out the top of a cave goblin (with topknot) at the back on the lefthand side.
Thanks, I've stuck with the ones AA have said are <20mm for now