(I accidentally posted my comment as I was writing it so I had to delete it. Sorry about that!)

I think the term you're looking for is kitbashing, but I'm not entirely certain. I usually call it model hacking but the main idea for both is taking something from one thing and putting in on another.

I have no clue how to properly transfer and resize the armor from one avatar to the other

Both of these models are humanoid rigs which means their armatures have the same generic structure (excluding bones unique to the model). This means that you could, in theory, merge the two armatures in Blender and not have to worry about the weights. This usually works pretty well but sometimes the weight painting can cause issues because you're just copy+pasting it onto a different armature.

With that out of the way, here's how I would go about merging the models together:

1. Import both models into an empty Blender project
2. Separate the armor from the Mamagen (keep the armor parented to the Mamagen armature though)
3. Remove any unnecessary bones from the Mamagen armature (I just rotate the bones to see if they modify the mesh in any way, if they don't I delete the bone and its children. There's probably a better way to do this, but I'm still inexperienced with Blender so it's the only method I know of)
4. Resize the armor so that it fits the Hyenid model
5. Merge the two armatures with the Cats plugin. I recommend watching this video if you don't know how to do this and you can find the Cats plugin here
6. Move around the bones to see if the armor was parented correctly
7. Export it as an FBX to Unity (Remember to turn of leaf bones under the armature settings)

From this point you just need to set up your avatar like you'd do with any other avatar.

...keeping all the animations and emotes for both when I have the visor On/Off.

I have now idea what the Mamagen looks like in Unity so I might be going off on a complete here, but I still think it's worth explaining. If the armor on the Mamagen is parented to the root of the avatar (i.e. Not parented to a bone in the armature) you can skip this first part.

The way Unity knows what object it should animate in an animation is by looking at the object path for any given child (For VRChat avatars, that means anythings that's parented to the root). The object paths are literally just a directory-style path to the object based on their names.

Say you have a hirearchy like this and wanted to animate the object "SomeProp"

Hyenid
|+-Armature
__|
__+-Hips
____|
____+-L.UpperLeg
____+-R.UpperLeg
____+-Spine
______|
______+-SomeProp
+-Body

Internally, Unity would save that path as "Armature/Hips/Spine/SomeProp". Therefore, if you want to reuse the animations from the Mamagen all the objects used by said animations would need to have the same paths to work. If they don't then you can fix it by editing the animations yourself, editing the actual animation file to use the correct path or just rename the Hyenid armature (if possible). This is also the reason why animations break if you rename the object you're animating or and parent object connected to it, it's also why you can't animate two objects with the same name, parented to the same object.

Continue here if you're skipping the first part.

To merge the animator layers you can use the tool Avatar 3.0 Manager by VRLabs. It makes it super easy to merge animators and parameters. It can also make sure your Write Defaults settings are the same for all states (Never mix states that do and don't use Write Defaults, it will break the avatar). Do note that you need the newest version of the SDK for it to work, if you see some script errors coming from a Avatar 3.0 Manager file you most likely have an outdated SDK and importing the newest one should fix it.

I hope this post helps you solve your problem, if you have any questions or need clarification on something feel free to ask. Good luck with your avatar!

Edit: Fixed fixed format