Is there an API to change player models without changing texture?

I can't use Kleiders because it requires a new texture in procedure blocks. I work with forge and don't know how to code at all. Until someone can recommend something I can use I'll try to finagle around with it if I can get it to work.

I want to make a mermaid tail for mermay but I can't change the player model to be legless while keeping the player's skin.