look at/shoot your own reference.
quick and dirty, you could shhot some fire, import your fire clip, blur it out crazily and scale it up so that just the firelight and not the actual flames would be framed in your original clip's size. So if your footage is 640x480, scale up the flame so that just a segment of it with flickering light is 640x480. Use the Add layer mode to add the fire to your original clip. A cheapy way to get the tint and randomness. But you won't get proper shadows dancing from the flame..really depends on the shot whether or not you can get away with it...
Your bg is rendered from 3d? You could 3d track the position of the light source - the flame - and parent a flickering light to it so you get a more "real" look. Put those 3d dudes to work!
hope this helps!
jon a