Hello, I'm going to play FNV with an average character with the intent of creating a patch for World of Pain so that it will be balanced when played along with Project Nevada. The creatures and npcs are all named 00CreatureEtc, so I can identify all of them from AWOP. I've seen actors with hundreds and thousands more health than I think is reasonable to play with PN, so I will be editing the health, attack damage, and level.

Now, what exactly does the level of the creature / actor do to gameplay? For example, Rex is a Level 15 half-sized behemoth with 1000 health, the same level as a standard behemoth with 2000 health. Does that mean that they both will earn the player the same amount of experience? Ghoul Reavers are Level 17, will they give more experience than a behemoth?

You could put them in the world, kill them and see how much EXP you get from each.

There's a bunch of gamesettings which determine what level ranges fall into which XP categories.