For example, if I have four Level 50's, and one Level 1 --- does the Level 1 guy gain the same amount of XP as the Level 50s? Or does the game somehow scale the XP so that the Level 1 guy isn't going to gain 10 levels in one battle with high level monsters? If there is some kind of scaling involved, what is the system? At what difference in levels does the game penalize the lower level characters in XP gain?
As far as I know, no. All XP gained at the end of a battle is divided evenly among all living party members. This includes party members of any level, even level-capped characters (who essentially "waste" their share of the XP). Therefore, if you can keep a lower-level character alive through battles on very difficult floors, it's not too difficult to quickly catch them up to the rest of your party's experience level.