Slime Size Model Help

I'm making an "Ender Slime" mob. It's exactly as it sounds, a slime that spawns in the end. But I'm having problems with the sizes. Right now, I have 3 sizes as 3 different entities with 3 different models as that was what some other post said to do. Doing that, I now have 3 different slime mobs. The first one always spawns as a baby slime that uses the built in slime model, with hitboxes that go from really small, normal small slime size, and normal medium slime size. Second one always is a medium size slime with a custom model, with 3 hitboxes as well, that are all correct for slime sizes (small, medium, and large, these are the ones i want) Third is the big slime also with a custom model, with the smallest hitbox being medium slime sized, largest being double that of the normal big slime.

When I kill one, if it has the larger 2 hitbox sizes, then it splits into 4 of the same model size but with the smaller hitbox size. 

So how do I go about adding 2 of the models onto the third one?