Several of these questions are answered on the Dwarf Fortress wiki, and there's a very handy modding category there that will link you to most of the essential info bits.
If you want a particular stone to melt, go into the "matgloss_stone.txt" file, find the stone you want (or don't want, as the case may be), and set the melting point to zero or something. This may have the side effect of making stone layers of that sort disintegrate even before mining, but I'm not sure. There are also ways of transforming any item into any other item. So you could, potentially, smelt stones into rats. This requires much more modding, however.
For bears, add the [PET] tag to the bears you want. If you want bears that can be trained into hunting/war animals like dogs can, also add the [TRAINABLE] tag. If they're not common enough for your likes, add the [COMMON_DOMESTIC] tag.
For dwarven lifetimes, find the dwarf creature profile in the creature_standard.txt, and change the values of "[CHILD:12] and [BABY:1]" to whatever you want. The values inside determine how long a dwf will spend in that stage before progressing. If neither tag is present, they will jump fully-formed from the womb (presumably, never modded it that way myself).