Multiple, Layered Entity Textures? Texture Heredity?

Hello all. I'm new to Mcreator and would really appreciate some help with an issue regarding entity textures. I'm creating a mod that adds koi fish with an emphasis on collecting and breeding different colors. I don't know any coding but I'm willing to try to learn. I'm also using Geckolib with Nerdy's Geckolib Plugin if that helps.

Part A, Different Textures: how can I make different textured variants (like axolotls, cats, horses) of one entity? I saw one post on this forum with some code to use for this, which I can probably work with, if base Mcreator options don't do this. The reason I'm making this post at all, then, is my other two questions, because I don't know if the code will work with this. 

Part B, Layering: I've made multiple pattern and color options with approximately 180 variants (though I'm not including all of them because some don't look good or realistic). Still, there are a lot of combinations. Is there a way I can layer entity textures so that I don't need to make a unique texture for each combination? (i.e. I make one base and one of each type of pattern in each color, and they can combine?) I'm guessing horses in base Minecraft have this feature?

Part C, Heredity: I don't know if breedable entities will pass on colors to their offspring by default, but I'm guessing there's more I have to do. I'm tying koi variant to multiple NBT tags: Pattern, base color, color 2, color 3. As of now they are all numerical tags--should I change this? Is it important to have one NBT tag for "variant?" I want koi to be able to pass on their pattern or any color independently of each other, the way horses can.

Thanks in advance!