I never used Mixamo, i just start learning to animate using 3DS Max, is really hard because i’m not an animator, anyway let’s see if this can help you:
you can use the same animation if the skeleton is exactly the same, If you are trying to apply a Mixamo animation to a different skeleton (like the mannequin) it will not work as expected, because you need to fix the pose of the mesh, set the bone name appropriately and also retarget the animation,
You always need to set a skeleton for the animation, so try to download for example one mesh with animation, and another mesh without any animation, and inside unreal engine (when you are importing the second mesh do not create a new skeleton but select the existing one) in this way the animation should work also on the second mesh without any issue (two different mesh that share the same skeleton and animations, this concept is working if all the characters have the same skeleton structure.