I'm pretty sure you can rank up fighting characters that are below your level. I say this because I am sure I did earlier this evening when I was playing. Okay, I reckon the 'exp' awarded for fighting lower ranked ghosts is lower than fighting equal or higher ranked ghosts, but it still seems to count.
Also, does losing a fight damage your standing? Can you be demoted? I'm currently standing at about 105/4 win/loss.
Call of Duty 4 - 1000/1000
Call of Duty 2 - 1000/1000