I've got one big mod right now that uses a lot of different creature classes, Spellcrafts;
SPELL_MAGIC_ESSENCE - implies the creature is an essence, consumption/being bitten by it imparts new CDI "spells" the creature has access to
SPELL_MAGIC_POTION - implies the creature is a consumable potion, granting non-permanent effects and abilities.
SPELL_ENCHANTED_INANIMATE - the creature is actually an/a number of inanimate object(s) enchanted somehow. Enchanting basins work like this.
CANNOT_LEARN_MAGIC - implies the creature cannot gain spells from essences, used in gnolls and komeng currently.
SPELL_MAGICAL - Implies the creature itself has/is composed of magic. Not used as much as it should...
SPELL_MAGIC_USER - I dont remember, I only used it on the master sorcerer megabeast and wizards, which both know innate spells.
MAGIC_IMMUNE - Implies the creature is immune/highly resistant to magical attacks. Used on gnolls.
COLD_IMMUNE, LIGHTNING_IMMUNE, THUNDER_IMMUNE - syndromes associated with these elements ignore creatures that have this, only currently used on elementworms (special megabeast I'm working on) and ice imps, because these are poorly implemented at the moment. Not sure why I didnt specify anything for other elements. fire immunity (and true cold immunity) is kinda only granted through material properties and doesnt have any associated syndromes so far...
FEMALE and MALE - used by transformations to attempt to respect the sex of the target creature when transforming them into the desired creature. Basically ensures a female dwarf gets turned into a female giant cave spider or whatever. Iirc they all default to female if these are not present on the target creature.
POLYPROTECTED - not used in the creature raws, but creature transformation interactions in the polybestia school respect it, meaning creatures of this caste would not be transformable.
SPELL_SUPERNATURAL - has some weaknesses to silver, and magic
SPELL_FAE - implies weaknesses to iron in any form, and dictates that the creature is a elf or fairy type.
SPELL_CURSED - the creature is the result of some form of curse, like fauns, which are a deity curse, and werebeasts.
SPELL_UNDEAD - it's undead, probably has no soul
SPELL_UNDEAD_SOUL - the creature is undead, but possesses a soul
SPELL_VAMPIRE - is a vampire, implies bloodsucking, vulnerability to holy wood and silver
SPELL_DEMONIC - it's a demon. demons are unanimously weak to adamantine.
SPELL_DIVINE - is a deity or angel unanimously impervious to adamantine, weak to fire glass and demonic metal
SPELL_ACHWURM - achwurms are creations born from uncontrolled magic. They're also weak to all forms of magic, silver, and adamantine.
SPELL_PLANTSHROOM - like GENERAL_POISON, but used on plants. diseases and interactions that should only target plants target this token.
SUMMONABLE - a body part from this creature is a valid target for generic summoning interactions (which are basically a corpse reanimation + transformation.) The following dictate which specific class of summoning interaction works: FIRE_SUMMON, WATER_SUMMON, AIR_SUMMON, EARTH_SUMMON, STORM_SUMMON, SYNTH_SUMMON, NATURE_SUMMON, DEMON_SUMMON, DIVINE_SUMMON, PLANT_SUMMON, NECRO_SUMMON, CREATION_SUMMON. So an earth summoner can only conjure using body parts that possess the EARTH_SUMMON creature class. This lets me prevent summoners acting like necromancers and just raising any old corpse immediately, requiring a two-step process where the corpse must first be transformed into a creature of the appropriate class, but is immobile and quickly suffocates, and then re-reanimate the body parts of that creature.
BIRTHING - explicitly used by the birth-sphere secret's self-transformation, a creature that has a melting-falling-off body part. This implies the creature is currently in that "giving birth" form, and like the summoning tags gives the reanimation interaction an appropriate target.
BIRTH_CHILD - likewise, but this one signifies the child of the above. It's actually a body part reanimation from the body part that fell off the "mother." I wanted these to be intelligent and stuff, but the fact they're a reanimated body part prevents this, because it would cause crashes.
VERMIN_HOST - used by a diseased vermin-carrying summoning special I was working on at some point, never actually made it into play... When I revisit diseases, that might change.
I use a couple others for other mods:
PET_WAR - implies the creature is war-trainable, used for civs I wanted to adopt any war-trainable critter
PET_VERMIN_HUNTER - similarly, implies it hunts vermin
PET_PACK - again, this time the animal can be used as a pack animal
Any mod that uses creature classes MALE and FEMALE on the appropriate castes for its creatures should thus work with Spellcrafts' transformations as long as they fit other criteria i.e. not POLYPROTECTED and can_learn... speaking of I just found a bug that must be preventing vampires from using transformations. Oops.
Similarly, anything that has CANNOT_LEARN_MAGIC will not be able to learn from essences, any creature with SPELL_PLANTSHROOM will be treated as a plant or mushroom type organism and be vulnerable to blight. COLD_IMMUNE will make it ignore frostbite syndromes, but material properties will have to be changed to prevent it dissolving/crumbling away at extreme cold temperatures.