caz: i love this!!
you're doing a great job on the atmosphere, the composition is really good and i think the fireflies will make a nice addition too
. about making the cat blend in better: photo comix has a trick that i know he's posted here on gc a while ago - couldn't find the thread right now, but i know it's here in the tips and tricks or tutorials sections. he makes a duplicate of the background layer, places it over the render layer (the cat in this case) and puts it on overlay. i've used this technique a lot since he tipped us off about it and it really helps making the renders blend in. i often tweak the overlay layer using a layer mask so the effect gets stronger on the render than around it and i smudge the layer over the render so i don't get sharp details of, say, tree branches over the render. another way is to just pick some colours from the background layer, paint them on a new layer over the render and put that on overlay.