Seeing a 1 HD pit fiend would probably be enough to make me throw away my dice. Possibly the worst idea ever, sorry dante but it's true. Could you imagine if the guy didn't get to CR 20... what's he gonna do? Take a job at Hot Topic? Or better yet, become a fixture for the picture for a Church of Satan book cover. I mean, a Pit Fiend is a Pit Fiend because it's the top of the heap, not because it's a race, like elves or orcs. What you're talking about is tantamount to seeing 'baby devils', which is anathema to the entire idea. The only thing I can see trying to rig up regarding this is a sort of age category for devils, based more off a combination of merit and age than age alone, with each category taking the creature up a notch in the hierarchy.