Hi RhythmScript,
Playing additive animations on Slots works well, and is something we do a lot on Fortnite.
Slots take care of applying additive animations, and mesh space additives as well automatically, so you don’t need to have a ‘Apply Additive’ node hooked up to that. It’s already built in. The input of a Slot is regular local space FK data, and so is the output. If you throw local space additive or mesh space additive animations to it, it will add them to your input and output the transformation.
If you throw at it different types of data, it will also handle the blend out properly between all these types.
Hope this helps.