I figured it out. I removed all <set xpath= lines except the /@max_level">100</set> line from the progression.xml in RussianDood's mod. I also edited the display_entry and CraftingTier lines so that there were six tiers. Finally, I made Pahbi's load before RussianDood's mod.
And because of all this I managed to discover that many of the item groups in Pahbi's mod needed a line that changed @level= for one of the items, as it wouldn't unlock crafting of the item until tier two, because they made the tier one of that item unlock before it was supposed to.