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You can create creatures from different factions, but it's more difficult to integrate them into a general list. Although there was a mod for the fourth column...
You can create NSFW creatures for yourself without sending them in, but keep in mind that there's plenty of space, and there doesn't seem to be any censorship, so you can reserve a spot or two for yourself.
To understand how to change creatures, icons, and so on, I recommend looking at an example from the developers or other modders. I'll just say that any creature starts in data/data.pak/GameMechanics/Creature/Creatures/faction/creature.xdb, and from there, many links spread throughout the archive; you can guess their purpose from the names.