If you lack a specific DLC, then you will be unable to play as the randomized religion that overwrote it. New religions are simply the base game religions but possibly given new names, icons, and/or features. Note: Randomized religions still require the corresponding DLC to play them.
They will be found in the same location as normal. Number of Dukes/Kings/Emperors - These all determine the relative frequency of the different ruler types in the world and can be set to None, Few, Normal, or Many. Number of Counties - This determines the maximum number of counties that each ruler can be given at the start and can range from 1 to 8. Random World is intended to create a start similar to the standard starts, but with changes to any combination of cultural and religious data.