Should be doable, but I can't find which layout file contains the character creation menu.

Yep, I see it.

Here's how I'd do it:

Step 1: Go play with the unit types. First, create one unit type per class that inherits from STARTING PET (e.g. PETS FOR OUTLANDERS). Second, use multiple inheritance to create a group of unit types that inherit from every permutation of the first group (e.g., PETS FOR OUTLANDERS AND EMBERMAGES). Third, go change all of the starter pets to use the appropriate new unit type. (You might structure it differently if you had no pets that are open to more than one but less than all classes.)

Step 2: In the layout, clone CharacterCreate/LeftSide/LeftPlatePet/LeftBackground/ActualCharacters/PetList until you have one copy per class. Change the allowed/not allowed unit types for each list as appropriate to get the pool of pets you want for each class.

Step 3: In the layout, modify CharacterCreate/LeftSide/LeftPlatePet/LeftBackground/ActualCharacters/PagingLogic to allow each of the cloned petlists to send the same signals as the existing one can.

Step 4: In the layout, modify CharacterCreate/BackButtonLogic so that the appropriate clone of PetList gets shown -- and the others hidden -- immediately after ShowPet runs.

I think that should do it, or at least give you a good starting point

I'm having no luck at all - its just a big mess in-game. I'll attach if you feel up to looking through it, but no worries either way. I'm just discouraged from banging my head against this lol