This is how I would imagine it in MCreator:
Mobstates would be toggled with procedures and AI tasks (except for baby mobstate), the mobstates would be made in procedures (with ability to change models, textures and bounding box) in mobstates, there would be a baby mobstate, that isn't set with procedures, it would just ask for a baby model if current entity is breedable.
Issue comments
I suggest you check our tutorials collection playlist on our YouTube channel which contains many examples and tutorials that can help you get started with MCreator: https://www.youtube.com/playlist?list=PLAeL-oIFIEngE6jRgFYeFMfuj8WQsO3Ei
This will be doable with NBT tags and this feature: https://mcreator.net/tracker/issue/49202 (conditions will apply to AI tasks too)