Personally I'd reconsider the dog idea. When we had a dog, she only went in our yard for her "business" first thing in the morning and when I got home from work. And of course she went out, if we went out. She just didn't like being out there by herself. Her exercise came from two long walks every day. I think the right breed could do well in an apartment.

I've never had small mammals, so can't help you there.

Veronica