I think the simplest way of doing this would be triggering another state once your normal movement state is done, where you’ll play slowing down / speeding up animations respectively (btw you can have both in the same state) and create blend spaces for each of these to adjust them to different speeds.
You can use the Layered blend per bone node, plug an animation or a locomotion (I refer to state machines by locomotions, and by plugging a locomotion in, I mean plugging the cached pose. You can create a cached pose by dragging the output pin of a state machine out, releasing it on an empty space and typing in “cached” on the menu that pops up, and selecting the New Save cached pose option.) into the base pose, then plug the animation which you want to only affect a specific part of the body into the input (Source) pin of a slot node (You can right click on an empty space, type in “default slot” and select the "Slot ‘DefaultSlot’ " option. Then click on the node that has just been placed, navigate to the details window, and under the Settings section, click on the bar next to the Slot Name setting and change it from the default one to your desired one.) and plug the output pin of that slot node into the Blend Poses 0 pin of the Layered Blend per Bone node. Also, to create a new slot for your character, you can simply open up an animation asset, navigate to the Anim Slot Manager window (if it’s not open, you can open it by navigating to the Window option in the menu on the top left and selecting it) and click on the Add Slot button.
That’s exactly the right way of actually learning things! You’re on the right track! 
I think the course I’ve provided is a pretty good starting point. (Don’t mind the leaning part of the tutorial, I just wanted you to check out the state machine part of it) You can add more things on top of it like creating a main locomotion (state machine); separating locomotions for ground movement, air movement etc.; using the Apply Space Mesh Additive node to blend your existing movement animation / blend space with that leaning blend space if you like to use that as well; and more!
I hope I was able to guide you on your objective 
of course there are more things to cover but I think these would suffice to get you a better grasp on the context. Good luck!