1) What was more beneficial in consuming the dogs body? The bone structure resulting in more leverage with shorter appendages? The muscle structure wrapping around the bones? Learning new ways to modify ear and nasal canals? etc.
TenSoon mentions the bones as being full of unexpected advantages, such as running faster, but I think the new ways to form ears and noses are probably something he can transfer to human bodies, so in that way they would be more versatile.
2) Could a kandra that has memorized human bodies use just about anything as a skeleton or does it have to be human bone? That is, could they create a skeleton out of sticks or metal pipes?
I think other materials would work, if they were the right shape and size and strength, and if they didn't disintegrate in the body's juices. I don't think kandra can carve bones into different shapes, they have to use what they're given.
3) Could a kandra store extra bones inside their new body so they could instantly change their size? Like, could they keep an extra couple vertebra, leg bones and arm bones, etc. stored where the lungs should be and then quickly go from being a kid to an adult?
Yes, I bet they could, though I'm not sure about storing them in the lung cavity. I think that in order to function as a human, a kandra needs to have the organs of a human. Remember when TenSoon had to quickly form a lung and larynx as well as a tongue, before he could speak? Maybe kandra could store a few bones somewhere else, though, like in the abdomen amongst the other organs. There would be a pretty strict limit on storage space, though, before the body would start having strange bulges and lumps.
4) What sort of size restrictions do they have for consuming bodies? Could they do a horse or is that too far to stretch their muscles? Could two kandra team up to do a horse, one being the horses....you get the picture.
I get the impression that OreSeur/TenSoon is the first kandra ever to try to imitate something non-human, at least as part of the Contract. On the other hand, mistwraiths use all sorts of bones, so maybe kandra do as well, when they're not working. I would think there must be a size limit, though it might be possible for a kandra to gradually increase its size or its capacity to digest. I don't think kandra could team up; it seems to me that each kandra needs to remain its own creature.