What thickness would you recommend?
There's no such thing as an absolute stickfigure thickness. It all depends on preference and style.
What you should be focusing on however are the basic principles of animation. I can see you're still having a hard time creating poses fluidly. The most prominent faults I see here are:
1.) Stop and Go. Again, when you animate poses transitioning to another, please just don't make them stop completely (Unless if the action calls for it).
2.) Perspective. Okay, when both a figure's arms are outstretched and you're looking at the side, they don't look like an equal sign (=) but rather, just the front arm showing.
3.) Lack of frames. A crap ton. I know you're aiming for slow movements and using double -> quadruple framing but please don't use this technique until you've actually mastered using the proper number of frames to animate a movement.
Personal Note: Eh, so far, I'm not sure what you're trying to get down with such short lengths.