Yes rocks are sentient beings. You will get as many sentient beings as many as you break. Every atom by itself has sentience. That is Quantum Theory for you.
I am calling the entire universe sentient. Physical/Non physical. Everything. To give you an analogy, whether it is a drop of water or a pond or a river or a lake or an Ocean, it is water and it remains water no matter how many divisions you do of it. Likewise no matter what form it takes and how many sections you make of it, it remains sentient because it is part of the universal soul.
There is no scheme but just say a journey through various facets of life. So becoming a human for an animal is not scheme it has to work towards, but something that happens when it has explored other forms of life. As far as humans are concerned, having achieved a level of consciousness where creativity is unbound to get back to an animal form would be sort of a retrogression or a necessity for the soul to fulfill some karma. I am no expert, the subject is still new to me.
Once I saw this documentary where a set of deep ocean fishes, small tiny fingerlings, where swimming in sulphuric acid pools. Got me thinking if this was hell and let us say some soul had to spend some time in sulphuric acid for say 10 lifetimes, this would be a nice way to do it. Become a fish for 10 lifetimes, say 2 months a lifetime, swim around a sulphuric acid pool, and then phew back to human life form after 2 years of penance.