One concern that comes to mind is if you use the SLC over cured concrete is the potential for cracking as the existing floor will be a cold joint to the SLC. I want to say there is a primer that will cure this aspect but I would look into rock board and thinset for a level surface to work off of.
Well, made the plunge and poured a 40lb bag of self leveler. Waiting on the dry now!
My pour was roughly 6ftx2ft. I was about 1/4" low in the back. I used 1 40lb bag, and it was too much. I probably would have used 2/3 a bag. I definitely recommend a frame.'
edit: After looking back at your OP, id say 2 bags would be almost perfect for that size, but how low are the spots that you are worried about. thats the key. If you only need a skim coat, 1 bag might even do it. If you dont have a mixing paddle, get one for sure.