Basically, I want to know if there's a way to safely combine two different leveling systems/mods, the XP system (from Oblivion XP) and the vanilla (or vanilla-like) skill-based system. I.e. I want to gather XP from kills and miscellaneous actions, but also have my skills level up by using them, both at a reduced rate to maintain a reasonably slow leveling speed, of course. Ideally, I would simply use Oblivion XP, and then use some skill rate modifier mod on top of it so that it takes control of the skill rates while leaving the XP system in place. Is that even possible? If so, what mods/tweaks to mods could one use (Progress, Fundament)? Thanks very much in advance!

Is it "possible"? Sure, probably.

Has it been done? No idea, but I think not. You'd probably have to construct it yourself, though I guess you could work on the Oblivion XP base.