To replace an animation, you can simply have the clip as a variable, and then based on the rules you want ( random of whatever ) you can specify which animation you want to assign.
Regarding switching the skeleton, by default you can’t do that, at least using BP, C++ not sure, since you do have a target skeleton in Class Settings > Class option > Target Skeleton ).
But if you’re using UE5 you can use the IK Retargeter, so that you can have a single “base character” and switch between different characters( with different skeletal heirarchies ) with few steps.
In this video you can see how I’m able to drive 4 characters with a different skeletal hierarchy using a single AnimBP.