Property "types" are handled by SlimeProperty instances. Whilst creating SlimeProperty objects is not allowed, there is a list of all available properties. Properties and their values are stored in SlimePropertyMaps.
Example Usage:
// create a new and empty property map
SlimePropertyMap properties = new SlimePropertyMap();
properties.setValue(SlimeProperties.DIFFICULTY, "normal");
properties.setValue(SlimeProperties.SPAWN_X, 123);
properties.setValue(SlimeProperties.SPAWN_Y, 112);
properties.setValue(SlimeProperties.SPAWN_Z, 170);
properties.setValue(SlimeProperties.ALLOW_ANIMALS, false);
properties.setValue(SlimeProperties.ALLOW_MONSTERS, false);
properties.setValue(SlimeProperties.DRAGON_BATTLE, false);
properties.setValue(SlimeProperties.PVP, false);
properties.setValue(SlimeProperties.ENVIRONMENT, "normal");
properties.setValue(SlimeProperties.WORLD_TYPE, "DEFAULT");
properties.setValue(SlimeProperties.DEFAULT_BIOME, "minecraft:plains");
// add as many as you would like