if you dont fancy mixing a load of verm, you can always use thermolite /celcon blocks layed on their side for insulation under the firebrick cooking floor........that the way quite a few of the pompeii builders in this parish have gone. The blocks are very lightweight and they can easily be cut/shaped with a panel saw.
I think the choice on whether to lay your hearth firebricks on a bed of dry homebrew or a wet slurry of it is entirely a matter of personal one. I chose the thin bed of slurry homebrew laid out with a notched trowel simply so that i could get the top layer of the floor nice and level. I dont think i couldve done that on a dry mix since the firebricks are not exactly uniform.