Civ with multiple [CREATURE] tags doesn't make them multi cultural (at least living together that is, unless new changes allow it) but adding more creatures to a civ will just pick at random what CREATURE will dominate that site, so a [ENTITY:ALL_RACES] would have a human town here, a elf town there, and a dwarf town elsewhere, but they'd all count as ALL_RACES.
Castes is the only way you can imitate a multi cultural race, but then you have the awkwardness of a dwarf couple having an elven child, a human elf couple having dwarf children.
Regarding pop_ratio if you wanted it 50% dwarf you will have to make sure that their number would be 1/2 where the /2 portion would be the ratios of the elves and humans
but the ratio calculation goes:
MY_CASTE_RATIO / (ALL CASTE RATIOS)
MY_CASTE_RATIO / (CASTE_A_RATIO + CASTE_B_RATIO + CASTE_C_RATIO + MY_CASTE_RATIO)
50 / (4 + 16 + 30 + 50)
50 / 100
0.5
now that is simple, but remember each race would have MALE + FEMALE castes but you don't have to have the numbers all add up to 100 I just chose to do that for ease of explaination in the end it could be 1/60000 for all the game cares that caste would never show naturally (unless by PUUURE stupid luck) You can create special leader castes that only hold positions of power by using the [CREATURE_CLASS] tags in the creature and the entity position