Mecanim Generic Rig

That is not good ... that means I would have to remove the bones from my rigging to make this work... I have accessories that are supposed to be rigid and it would be a total waste

Im using generic in my project.  It works fine

I've only seen the options in the character file drop down for Humanoid and Legacy. Are you saying you can set the fbx rigs to Generic then use Humanoid or Legacy in UFE character settings?

Unity Humanoid quality is okay if you really need to share animations from character to character within Unity but in order for it to work perfect they still need to have the same skeleton hierarchy and rotation axis. The down side is your animations are not one to one. Humanoid has degrees of freedom setting for the bones that will limit the bone rotations regardless of what your animation does. There are setting for it but I've never gotten them to do what I want.

For example if you have a sub object in your FBX of a melee weapon that's not part of your character skeleton hierarchy and in your animation it lines up with your hand. When you put it in Unity with Humanoid as the rig the hand and melee weapon wont line up and they'll be offset.

UFE supports legacy in the character file and this is what I've switched to using. My animations are 1 to 1 and any sub object I have as part of my character lines up where it should.

I don't have UFE setup here at work so I can't check but from the setting in the character file it doesn't look like it supports Generic. It would be nice if it did. So, I hope I'm wrong. Generic is nice to keep your animations how you animated them and for non-bipedal rigs.

Legacy works but Unity recommends not using it.

Twists should still work if you're using Humanoid. As long as the animation on the twist bones are baked when you export / save the animation to fbx from w/e animation package you're using and the twist bones are included in the mask in Unity.

Already done... in the humanoid setup, all of the bones are set in the transform mask... another issue too is the characters bones will not always be the same size... like in my game Blade is 6'5", Aiko is 5'7"... I can already see issues when shorter characters fight taller ones. And I get a warning that the retargeting might not match up because the bones are rotated differently. I may end up in the animations making sure it uses the main avatar.

For the alternate model, it mentions using the same avatar as the main model... so I hope to see this.

The names don't have to be the same as long as you're using Humanoid. Humanoid is a control rig over top of your skeleton similar to MotionBuilders control rig. You would just need to setup Avatar Definition with the bones for each character. Then you should be able to apply one characters animation to another. But this will give you mixed results depending on how different your two rigs are.

That shouldn't disqualify it. As Chrisb3d said unity's humanoid solution is pretty versatile. Have you tried  any of your rigs as humanoid? Some of the complex human rigs out there aren't compatible because the have too many extra bones.

Interesting, those extra twist bones dont break it?! I didn't realize you could skip bones in the hierarchy. Good news for me actually. I knew extra bones outside of the humanoid hierarchy didn't break it but I didn't realize you could have extra bones within the hierarchy.

I'm using humanoid. You can have extra bones in your rig. The point is you need to have the Avatar set up to tell it what the bones are.

So think of the Avatar as a translation system.

The model is told from some animation that "The left forearm is supposed to move 3* upwards." and it goes and processes "Hmmm, the left forearm should be character_larm as the user designated in the avatar file. So move character_larm 3* upwards".

Then you have an Avatar Mask which you set up in the animation to say "If you get references to these bones that are not labeled in the Avatar, move them anyways otherwise ignore any calls to them." So animations I've made specifically for one character I have every option checked in the Avatar Mask and every animation has that avatar mask linked to it along with linking to the initial character's Avatar.

Just for the record the hit boxes do not need to be attached to bones. My weapons have empty child gameobjects placed where I want my hit boxes to be. No worrying about offsets.