Ohhh man. I haven't done a redline in... too damn long. I hope it doesn't show too badly. I know for a fact that I'm a little out of practice, so sorry if the anatomy isn't quite on par to my usual work.
I'll start off by saying that I love the loose, sketchy style you drew these in. I tried to keep my redlines in a more cartoony style to match how you drew the originals.
One of the first things I noticed, especially in the strutting cat, is how straight you drew the spine. One of the most striking features of cats (to me at least) is how curvy their spine is. Even when they're standing perfectly straight, their spine has a nice curve to it. When drawing cats in a more stylized manner, it's important (and really, really fun!) to emphasize this trait. It'll help make poses look less stiff and more organic.
Another thing I noticed is that the front legs on the original drawings are really skinny. This may be a stylistic choice, but make sure that the legs look like they could support the weight of the animal. Also, make sure you're paying attention to where the joints are in the legs. Even when the legs are straight (like in the strutting cat and sitting cat) you should still be able to see where the elbows are.
On a similar note, keep in mind that a cat's neck attaches to the back of the skull, not the bottom of the skull like on a human.
You seem to have a pretty good idea of how long the body should be, so nice work with that! A lot of people draw cats with bodies that are way too short. However, be careful not too make the spine too long when the cat is in a strange pose; the cat on the top's body is really long. Try and imagine how the body would look if you rotated it into a normal, straight position.
Also, the front legs on that cat look a bit short. I'd recommend sketching out a simplified skeleton before adding details to help keep proportions correct and consistent.
Don't be afraid to use references if you're not already. In fact, I highly encourage it! It'll help you with tricky things like drawing the face from different angles and working out the proportions of the legs.
I guess that's about it! I hope you found this helpful. Please let me know if you have any questions or critique; I'm always glad to hear them.
Some nifty references ~
Skeletal reference: [link]
Cat walking, side view: [link]