I have this custom character made in blender that is rigged, when i import this model to mixamo after I set the chin, knees, etc..., it works fine but when i import the original model to unreal engine it apears in the wrong pose anda some console warning.

But my questions are:

Mixamo needs a pre-rigged model to work?

and

Unreal needs a pre-rigged model to run the mixamo animations?