I am using the default Unity Pipeline
That's good news in this regard, as there are more spine-unity outline shader variants available for it.
I am looking to have only the outline showing while having the rest of the skin transperent so I can use it to make an outline only around a specific body part by laying it on top of it.
Having the outline on top of your hightlighted body parts will make your life harder than it needs to be. While you could do this, you would need to use the provided outline-only shader for it (with enabled Z-Write) as shown in the
Spine Examples/Other Examples/Outline Shaders example scene. Requiring enabled Z-Write of the outline shader however will result in a more jagged outline where overlapping parts cut the outline (harsh transition to fully transparent), due to the nature of how the z buffer works. You can see this in the
Outline Shaders example scene as well.
As a result, we would rather recommend to use the normal workflow of having the outline behind the highlighted body parts. Is there a reason why you haven't chosen this way, did you run into any problems with this workflow?