Well as far as it goes no one has ever added a way to set those things up.
To set up a pet store you need to have two rooms.
1200 (A shopkeeper is put here that sells pets).
1201 (The room you place mobiles in that you want people to be able to buy as pets).
After that its as simple as going to the store and buying you one.
If you want to be able to set them (like you asked) you will have to add code in mset so you can set those.