Stuart's anthropomorphism is the result of lab experimentation.
- Well, scientists have using white mice in experiments for years.
- His parents were the ones who were experimented, and passed it onto him.
Snowball and Mr. Tinkles are from the same litter.They're both the same breed, and have the same voice and temper.
Humans can understand Snowbell when he talks.The Littles just ignore him most of the time because he tends to be a whiny Drama Queen. I thought this because of a scene from the end of movie 2 - when the Littles are headed home, Snowbell whines that he's sick of never getting any attention and claims that he's not returning home with them. Cue Mrs. Little calling out to him (in a perky but slightly annoyed tone) that she'll make him dinner when they get there, prompting him to join them. Additionally, Margolo can talk to humans and no one seems to bat an eye about it. It's possible that sentient talking animals are commonplace in this universe.